leg kicks helps mix things up and keep the opponent guessing. a low leg kick may not be as deadly as a right hook to the jaw but it wears down the leg if continued and can take out the spring of the opponents leg making his movement less sharp.
Body kicks are very power and generates alot of power that can stun or knock out an opponent. The last roundhouse kick i remember in ufc is franklin vs hamil, and in terms of spinning heel kick, it was cung le vs...scott smith.
High leg kicks.. i dont need to say anyting bout that, just watch cropcop!
The vunlerable side of kicks are you can be taken down if you get your legs caught!