It does not boost performance. Normally frames are scheduled once per display refresh. There's no point generating frames faster than the display hardware updates - they won't be seen, and it just wastes power.
Unlimited framerate mode instead schedules frames as fast as possible. It's for performance testing. However since it uses loads of power generating frames that will never be seen, it will needlessly drain the battery on laptops and mobiles, so you should never publish an actual game using that mode.
That's pretty much all there is to it.