The only reason framerates gets limited is a feature called "vertical sync". The reason for it is that if your fps is far beyond your monitor's refresh rate, then there's a high chance of getting screen tearing which looks bad. While you can set your refresh rate in Catalyst, in the 3D tab there's an option for Vsync where you can set it on, off, or allow different applications to choose for you.
The reason to not use Vsync is that it artificially limits your GPU performance, which instead of allowing a steady 60fps (let's say you can achieve 75fps) it will still dip down to slower minimum fps, say 45fps. So long as there's no screen tearing, it's generally better to leave Vsync off.