The FPS capping is for not using processing power for no reason; I have ~2000 FPS ingame and the fans start spinning like crazy unless I go and limit FPS somewhere else. Having a V-sync option works too as it'd limit to the screens refresh rate, but I don't like v-sync for reasons... So a limit FPS slider would be nice but I get if you'd not want to bother and just have V-sync.
For example if you're running it on a laptop you'll drain the battery quicker for no real reason at all.