DirectX was created in the late 90's by Microsoft specifically for games to have more direct access to hardware. Back then the big thing was 3DFX VoodDoo and Glide (if any of you remember). DirectX for graphics wasn't really a big thing until 3DFX died and DirectX 9 came out. Anyone remember the ATI 9700 Pro? It was awesome for several years, right up until NVIDIA release the 6600 and 6800 cards (some were bundled with Doom3). Anyway, enough walking down memory lane...
OS X doesn't need DirectX in order to run games, it needs games ported to it. If you need to play Windows games, then you need to dual boot to Windows via Bootcamp. It's a good idea to wait until Windows 10 is released officially and install that. Until then, just use 8.1.