I put a left over PSU in my external box.
The major reason was simple. Testing and filling. With an external box, you usually have more hose, etc.. so it takes a bit to fill the whole system. Having it connected to its own PSU is a life saver, You are turning it on and off a lot during filling, having it on its own PSU that you can turn on and off at will is a great feature.
There are 2 ways to mod a PSU so it doesn't need to be connected to a mother board... one way is to use a paper clip (you can search the forums for that one) or you can twist one black and the green wire together to jump the mobo connector.
Heres my work log where I did that
http://www.hardforum.com/showthread....3&page=1&pp=20
and heres my external box, and where i modded the PSU a little more
http://www.hardforum.com/showthread....ight=bradyapba
i hope this helps!
THe PSU i had left over was a cheap 500w that came in my $35 broadway.com case.... so it was a perfect use for the PSU.
Like the others said you could buy and old AT PSU, those work too.
Chris