The air goes around it, cooling your hard drive as it does (good thing
A rear exhaust is good. It helps pull the hot air out of your case.
As was stated above, a blowhole is good as well. A blowhole is a fan and hole in the top of the case that pulls hot air out. Heat rises and gets trapped in the top of the case. The PSU pulls some of this out, but in doing so it doesn't get much cooling. A blowhole fixes this problem.
My case is a CoolerMaster Wave Master. It has the optional blowhole (in place of the top USB ports). Keeps my case cool, but I have to use a hub for USB attachments.