You will not telegraph the moment of your attack
This is the same reason for Taekwondo, western boxing (think Ali), or karate. If you are in a static stance, you have to shift weight before being able to gain momentum and cover distance. If you bounce, you can subtly change your stance and attack each moment you are on the way down as soon as you touch the ground.
Especially in point fighting, there are milliseconds that decide whether you make a point or not. When bouncing, you are highly agile and able to move across greater distances with each bounce compared to a shift from a static stance. That way, you can get into reach virtually every bounce and are a constant danger to your opponent. The only alternative is a combination, which inadvertently will leave you open to a well-timed counter-attack.
In higher level competitions, that looks rather silly because they bounce around all the time without anything happening, but what really happens is a constant play of (breaking the) rhythm and setting the distance. The goal is to hit the ground in a moment when you are both in striking distance and your opponent is already on his way up again, so that they cannot react and evade your attack in time.
If you think it through, it is really similar to Judo's struggle for the grip: In higher levels, it looks like nothing is happening to the outsider, but they are constantly doing subtle pulls and pushes and shift their weight to gain a superior position. And then, suddenly, they explode into motion. It is the same for karate point fighting: Since all that matters is that one point you are trying to get (and avoid), you try to move yourself into the optimal position before going for it. If you'd be static, your bouncing opponent would have an advantage at that, given he knows his attack distance and is able to read your posture.
Sweeping a fast karateka is really hard since it is hard to get hold of their arm and without it, you will never move your back leg fast enough (or shift weight on that leg to use the forward one) to sweep that foot.
And they do avoid attacks by bouncing. All the time. It's when they bounce backwards a bit more than usual, which means they are leaving the striking distance of their opponent again who just covered some ground. By adding these few inches to their usual bounce, they avoid an imminent attack. So it is really easy to miss that.
As a judoka by heart and constitution, I always thought it silly to bounce and never got used to it. But I have to acknowledge its merits when it comes to hit-and-run games in point fighting. Generally, the harder the style (and rules), the less people bound. Since stability and grounding becomes relatively more important than a one-off hit.