Well, externally it would mean you don't have a mains cable going up past audio cables and the like.
A bit neater, and lowers the CoG. Other than that, no real advantage.
( Dave Trouser ; the people's choice - 75% agree, Thu 4 Jun 2009, 19:04, archived)

it makes it easier to tidy up the tangle of wires inside the case too
if you're bothered about that kind of thing anyway
( well, Thu 4 Jun 2009, 19:06, archived)

oh wait, you already said neater

( well, Thu 4 Jun 2009, 19:07, archived)

The folks at Silent PC Review seem to go on about it
as if there were some thermal advantage, I can't see it myself.
( Moon Girl Technologies horrendous beanbag, Thu 4 Jun 2009, 19:07, archived)