Vsync caps the FPS to your screen's refresh rate and divisions of it, so on a 60hZ monitor with Vsync enabled you can only have 60, 30 or 15 FPS (at least that's what i remember from reading posts by the resident forum gurus). This however is something that can be worked around by enabling triple buffering, which once again enables the game to run whatever FPS it can.
All in all, FPS limiters are useful because they enable the GPU to split its workload in easier to manage "slices": if the card knows it needs to keep 30-40 FPS then it won't devote extra resources to render 60 FPS and that gives it enough time and spare resource to process other things that might crop up, instead of running at 70FPS when you look at the sea and then bogging down the moment you turn your view to look at a forest or ground objects
