well, in that case that I bought, it came with a fan on top that blows air out and there was no air flow coming INTO the case so I installed a fan to suck air into it to compensate for the air its blowing out.
I was getting dust bunnies buiding up in the little spaces in front and other places that air could come thru. I didn't want air being sucked thru the cdrw drives bay doors and getting dust on there and ruining the drives. make sense?
edit: I also installed a small 7.5 cfm fan onto the vid card heatsink cuz it didn't have one on it. its not in those pics