As he said, that goes on the bottom of the PSU and pulls air up into the PSU, where it is vented out the back. PSU fans, in addition to cooling the PSU itself, pull a lot of hot air from the case. In ATX and similar cases this is vital because they're housed in the top, where the heat will want to go. This is why many OEM boxes can squeak by without seperate case fans.
Oh also, a PSU being "new" doesn't necessarily mean it has two fans. Some of them are too cheap. Some don't need it. Some use one larger fan instead. I would take a single 120mm fan over two 80mm fans any day. Although some have one 80mm for the back, and a 120mm in the bottom.
As for opening a hole in the top of your case to vent out there... that's more for modders, and you wouldn't usually want two competing exit paths out of the PSU, so you might close up the back too. But that's still kind of stupid and only for hardcore case modders anyway. I think the time would be better spent ducting your CPU fan or some other interesting work.