My PSU has two fans--one on the front, which puts it inside the case pointing toward the front, and one on the back, which puts is on the outside of the case. What's the point of a fan on the back of a PSU? It's blowing air not onto anything in the case, but toward the wall. (If it were on the bottom of the PSU, I would understand--that would put it almost directly above the CPU.)

The fan is blowing air outside the PSU and doing so it extracting heat from there, nothing pointless.

Its usually more efficient to suck air through the unit then blow for cooling. Not sure what the purpose of the blowing one is doing. But not that you have mentioned it I have seen PSU with two fans....

I know its common to use two slow rotating fans instead of one faster rotating to reduce noise, but I don't know how that works out when they are placed in "series". Just a thought that popped into my mind

Effective cooling doesn't come from a fan blowing on something, it comes from creating a steady circulation of cool air through of the case. This usually means setting up at least two fans, one in the front to draw cool air in, and another in the back to eject heated air. Other fans can be used to ensure that specific areas get enough circulation, particularly the one on the cpu itself, which usually produces the most heat. The optimal locations of the fans may even be counter-intuitive, depending on the interior geometry.

The fans on the power supply are usually a part of this circulation system, in addition to providing cooling for the PSU itself.

Originally the only fan in a PC case was the PSU extract fan; all the other components were adequately cooled by the PSU drawing air into the case and across them; the warmed air exited through the PSU.

Now we are trying to warm the Earth with our computers (and succeeding too well!) we have many more fans to remove the heat.

It doesn't make much difference whether heat is removed by extracting air from the case or pushing it in -- it is the airflow itself that does the cooling.

Fans that push air into the case tend to pressurise it slightly. This is a better scheme than using extract fans which create a slight vacuum in the case and so draw air in through all the small gaps, bringing dirt with it (it's easier to filter the "push" fans' intakes than to put filters on all the small gaps).

For these reasons and to reduce noise, I removed the PSU extract fan. Details in this Silent PC Review thread.

I beg your pardon, but I don't use my computer to deliberately cause so-called global warming (I don't know what else you could have meant by "trying to warm the Earth"), and I don't know anyone who does.

So it can fly forwards and reverse?

The one on the back is blowing air towards the wall, which is actually a good design, IMO. This means it will suck air from inside the case, move it across the PSU components, then out of the case.

The one on the front may not be such a good thing, and I have no idea why they would put it there.

So it's something like:

?

Please don't take it badly -- I was commenting (intended humorously) on how the power consumption of PCs has risen dramatically since the early days (although there are some recent moves in the opposite direction such as Intel's Atom).