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Originally Posted by KnifeMissile
Even if the case doesn't have an exhaust fan, computer enclosures are not air tight, even if you disregard the exhaust valve(s) that it will certainly have. Thus, the air pressure in the case will equalize and, thus, the same amount of air that was pushed into the case will have left.
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But it can still leave slower than it can come in. Look at a soaker hose to get an idea of how this works. You can open up the faucet, and the water comes in quickly until the hose is full. Then there's lots of pressure on the intake side but, even though the soaker hose is not water tight, it still comes out slowly. Now, add a heat source to the soaker hose (leave it out in the sun) and wait about half an hour. Check the water coming out of the soaker holes. It'll be warm, even though the water going in is cool.
It works the same way for a computer case. If you put, say 300CFM worth of fans in the front, but the case can only evacuate 75CFM out the back, then you'll get higher air pressure inside the case than you normally would. It'll eventually equalize, but at a higher pressure than when it's off. And that extra air will just heat up instead of removing heat from the case. That's why you want a fan or two in the back - to speed up the egress of air from the case.