What does vsync do to begin with? if it increases FPS by turning it off, whats the point of having it on? 
Back in the early days of crt monitors it took a not insignificant period of time for the electron beam to be physically blanked out so it could be repositioned to start the next frame. This was known as the Vblank time.
During this Vblank period nothing is being drawn to the screen, thus it is the ideal time to update the frame buffer (the video memory that holds the representation of the current pixel data). The graphics chipset sends the vsync signal to notify code of this event.
Although most no longer use crt monitors, it is still desirable to not write to the frame buffer while it is being drawn to the screen so as to avoid the visual artefact known as tearing.
Thus vsync remains.
A natural consequence of vsync is that it limits your frame rate to the refresh rate of your display. This is no bad thing, as there's no point rendering frames that will never be shown to the user.
In badly written games, vsync can also cause input lag. (if the game polls input devices in the same thread that it waits for vsync)
Vsync can also cause juddering if your computer isn't fast enough to reliably render a complete frame in time for the next vsync signal.
