In all martial arts settings, it is best to mix up the kinds of techniques you are using. If you just throw roundkicks, your opponent will know to block the side of his body. If you just throw sidekicks, he will know to cut off to the side and execute a counter attack. So mix it up! A jab is a simple way to distract your opponent without compromising your position or your balance. The jab-kick combination is ideal for kickboxing and mixed martial arts fights, as a jab will often result in your enemy shielding their face...which means they can't see your kick.
But I can't use this in Olympic Sparring Competition!
Yes, you can. As long as you are not punching to the face (illegal in the current rule) it will be fine. It is a major deterrent to any Taekwondo fighter who likes to rush in. Few people would risk a jab without gloves to the face just for a point or two. However, if they are foolish enough to still try this and you don't want a foul (or maybe you're just a nice person), you can switch the jab from the upper area to the hogu. Even a light directed strike to an aerial opponent's torso can unbalance them, stop their motion, and cause them to fall over....and you get points for it!
There are other advantages that come with utilizing the jab.
Every time you throw that jab into the air, it does two things
A. It distracts your oponnent. A small object moving quickly towards their direction (the fist) will get their attention, regardless of how far away it really is. A distracted opponent is usually at least a temporarily unguarded one.
B. Every time you jab properly, your lead hip turns over just a little bit. Your jab masks this movement by distracting your opponent. Simplified, this means that while they're looking at your fist, your hip is turning into a lead leg roundkick or a fastkick.