This is a good question. And to answer it, you have to understand why karate kata (forms) exist, what their original purpose was, and how kata practice differs from sparring.
The original purpose of karate kata was to pass on self-defense technique to students. Each self-defense technique consists of one to three movements strung together in the sequence of the form. When you've learned classical jujitsu, you can begin to notice what's going on in the kata. The movements are the same, more or less. Those "blocks" in the forms aren't really blocks. They're grappling-based self-defense maneuvers.
What self-defense scenarios are we talking about? Well, they're the same ones that have always come up in real life. For example, a guy gets up in your face and grabs your lapel in one hand and raises his other fist up threatening to punch you. There are probably a dozen techniques shown in karate katas for dealing with this situation and situations similar to it. It's because it happens so often in real life.
Other scenarios: Someone has bear-hugged you from behind. Someone is choking you from the front. Someone has you in a side-by-side headlock. Someone is grabbing your shoulder from behind to turn you around so that they can punch you in the face with a hook punch. Etc.
All of these are typical self-defense scenarios that came up in the past and still come up today. Nothing has changed.
And when I say that the self-defense shown in the kata is grappling-based, I mean it. The blocks you're taught in karate are actually doing things like grabbing someone and throwing them, performing a standing arm-bar, bending someone over so you can strike to the base of the skull and shove them to the ground, grabbing someone by the hair and pulling them down by it, shoving someone over your leg to trip them, and so on.
Bottom line: It's jujitsu.
On a side-note, if you want an example of what I'm talking about, I posted an analysis (also known as a "bunkai") of a series of 3 movements in the karate kata, "Heian Sandan", at the following link:
Kata were developed as a way of more easily remembering self-defense techniques. And for that purpose, they did well. At the time books weren't very easy to make and produce. Drawings failed to show all the details, also. And they certainly didn't have video tapes and DVD's to record everything. So they relied entirely on memory. And kata made that part a lot easier and more reliable.
Classical jujitsu taught self-defense techniques individually, with a partner. And it still does, by the way. That means that you have to remember a list of techniques, and you have to have a partner with you in order to practice them. It was easy to forget a technique that way. Karate kata can be practiced in solo, which lets you practice much more often. It can be drilled into you so that it's much harder to forget.
The shape of the kata, and where it starts and where it ends on the floor also helps to remember the form. If you forget something, it will be more obvious that way.
And so originally, karate was supposed to be taught as a series of self-defense techniques that go along with the movements in the kata. You would learn both at the same time. Or you might learn the kata and then learn the self-defense interpretation after that. And the kata helped you remember them. That's it in a nutshell.
Now, why don't you know this? Why do we see countless karate schools teaching people to do kata in solo and almost never see any partnered activity working on the kata's self-defense techniques?
Well the answer is: Most karate instructors don't have any knowledge of it whatsoever. Less than a percent of karate schools teach this. These can be 8th dan black belts. They still don't know the first thing about the self-defense techniques of the kata they teach.
Why is that? It's because they learned from someone who didn't know it, and that person learned from someone who didn't know it, and so on.
You can see this more clearly in Shotokan Karate and its derivatives. It's because the founder of Shotokan Karate, Gichin Funakoshi, taught it as a form of exercise to Japanese school children as part of their official physical education program. In doing so, he deliberately left out the self-defense meaning of the kata. He didn't want any kids hurting the others with it.
So the self-defense meaning of the katas was left to the students' imaginations. Which is why you see a lot of really terrible interpretations of kata. I've seen people explain, "Oh, this technique is simultaneously striking a person in front you and blocking a strike from someone else behind you, whom you can't see." Yeah, have you ever been able to block punches you couldn't see?
Shotokan karate gave rise to the karate phenomenon in the second half of the 20th century. Prior to then, it was an obscure art practiced by a small number of people on Okinawa.
Shotokan became very popular in Japan because of it being taught in the elementary and high schools there. The children would graduate and form their own schools to continue practicing what they had been taught in high school.
It spread to Korea to become Taekwondo, Tangsoodo, and so on, because Japan invaded Korea in the early 1900's and forced its young men to go to Japan to become soldiers where they were exposed to Shotokan karate. They brought it back to Korea. And Taekwondo spread like wildfire.
After World War II, American G.I.'s were stationed in Japan and were exposed to Shotokan karate. They brought it back to the U.S., and karate soon afterwards overtook Judo in popularity.
Shotokan and all the hundreds of styles derived from it spread all over the world in the second half of the 20th century. Today, most Japanese and Korean karate schools can trace themselves back to Shotokan karate. And that is why most karate schools you might run into don't have any idea what the self-defense techniques are in their kata.
As for sparring, it's completely different from the self-defense scenarios I spoke about. Sparring is free-fighting. Nobody is holding onto the other person. Kata, however, is grappling. You can't learn to block from solo kata practice, nor can you learn to deal with a bear-hug from sparring practice. Both teach completely different things.
Now, back to your original question, which deals with how best to spend your time...
Since the vast majority of karate schools don't have a clue about the self-defense meaning of their kata, the only thing they can do is practice performing it over and over again in solo. They don't know why they're doing that. They just do. They mumble something about it training "proper movement", physical development, or maybe how it's supposed to teach your subconscious mind which will be able to apply it perfectly without thought when the time comes.
If I was given those kinds of answers for why we're practicing karate kata in solo for 50-90% of the class time, I'd laugh. Well, now I would. I know better now.
At least sparring is useful. You can see yourself improving. You learn distancing, timing, the ability to block and parry, how to evade and move, etc. Good stuff. But endless amounts of solo kata practice without any partnered self-defense practice? It's pretty much worthless.
By the way, that's why some karate schools (especially Taekwondo) only do sparring and physical development instead of kata. It's because they see those other things as useful. They don't know why anyone would want to do kata. In those schools, you learn kata so you can test for your next belt. That's it.
If your school is only doing solo kata and physical development but not doing sparring or any partnered self-defense activity, then you need to ask your instructor about it. Plenty of schools are like that. They truly believe that kata is "everything". They just don't know why. But they're going to repeat those kata over and over again until they reach the enlightenment they know someday they will achieve!
My advice to you is to go have a good look at other schools in your area. Find some Okinawan karate schools, for example, instead of Japanese or Korean karate. You'll probably have better luck.
Better yet, take up Muay Thai. And while you're at it, study classical jujitsu or Gracie Jiujitsu. That combination will give you what karate was originally designed for, and it will do it much less time and will be a lot more refined, effective, and useful. It gives you all of the knowledge and skill with none of the bs.
Hope that helps.