The beauty of cap space is that you can acquire players via trade very easily when you don't have to match salary.
Imagine Atlanta fails to land DH12 and looks to rebuild an expensive under achieving roster. The C's could make a play for Josh Smith and offer picks in return. He's no prize, sure he's a hot head, but a young athletic player who could be useful. Or Al Jefferson, also going to be in final year of an expensive deal, Utah could do worse than flip him for some picks, they won't get anything when he leaves in FA (same with Smith) and they are rebuilding.
Point is, cap space allows you to go after all players, not just free agents..