I'd guess Othello wants to limit an inconsistent FPS.
I locked Halo PC's FPS to 30 (a menu option, why can't all games have such?) because otherwise i'd get 60 in one moment and on the next its 45, 60, 50, 30, 32, etc... that's pretty darn irritating. Same with FO3... only that i can't lock the FPS.
Vsync doesn't actually lock FPS to refresh rate... it maxes the FPS at that point. Didn't turning of Vsync cause "tearing"? IE the screen "splits"