While my background is Karate, rather than BJJ, hopefully this answers at least part of your question.
The first thing to remember is that this is a specialised technique. As I tell my students: 90% of the time, you'll be using the first 10% of the syllabus. That is, after all, why we teach those techniques first - they are relatively simple to learn and use, and widely applicable to most situations. Spinning kicks are not part of that 10%, and most of the time you won't (or shouldn't) use them.
Of course, since they "look impressive", they are frequently given disproportionate prominence during demonstrations and showcases. Usually, you will be using this to improve or demonstrate your control, flexibility and skill, rather than as an actual attack. That said, this isn't purely an exercise - it does have practical applications.
The second thing to remember is that (outside of basic training) techniques do not exist in isolation. A spinning kick is rarely something to lead with - it is, however, something to lead into, or to follow up with. Here's a very basic scenario:
You throw a straight kick forwards, but your opponent steps back out of range. As your foot comes down - and while their momentum is still "away" from you - you push it slightly across your body, then twist your momentum into a spinning roundhouse kick
Now, because your opponent is still reacting to your previous (failed) attack, they are unable to change direction fast enough to capitalise on the brief opening. In addition, the reach of the roundhouse kick means that they are not longer - as they had thought - at a safe distance, but completely within your range. And, by folding the momentum from the previous kick into the spin, you make the entire move faster. This isn't just a case of learning to throw the spinning kick correctly, there are several secondary skills involved.
There are a number of movements that can "lead into" a spinning kick - even a simple block across the body with the opposite hand - but you need to be able to judge both how fast you can land the kick, as well as how long your opponent will be open or unable to counteract it for. As with anything, you have to both spot an opening, and recognise which moves can exploit it.
In a "self defence" situation, you generally won't have either the need or the occasion to use a spinning kick - keep it fast, simple, straightforward, and maintain solid footing. Similarly, MMA fighting spends most of the bout at a range which is unsuitable to apply this technique. However, when the opportunity presents itself, these techniques are effective - for example, the 2004 Olympic Taekwondo Heavyweight final, which was won by knockout. (The technique there isn't actually a good 'clean' example, as the kicker didn't manage to land properly. On the other hand, they hit the floor conscious, unlike their opponent - which one-on-one is of greater practical benefit)
While I could go into far more detail about when spinning can be good (and not just for kicks - remember, a spin doesn't have to be on the spot, you can travel at the same time), versus when it should absolutely be avoided (hint: this category is a lot larger than the other!), the best way is for you to learn is to practice - at first against the air, and then say to your sparring partner "I want to try something, could you please come in with this for me to counter". You'll get a feel for "this won't ever work" versus "I'm not yet fast enough for that" or even "I want to try that in a normal spar".
So, even once you've "mastered" them (and isn't that a loaded term - say perhaps instead "learned the basic form") you should only use them sparingly.