Silky my little Muppet. Jet A1 is a bit heavier than air and doesn't run uphill at all well, even in a pipe. So you need a pump of some description. Now the APU has a fuel pump as well and this can "suck" a bit - but not a lot. Certainly not enough to get fuel all the way from the wing tank to the tail (even in a dinky little A320). It needs a positive head pressure to function (cavitation is the word you might be searching for here).
So there is a DC pump for when there is only DC power - battery start for instance. However DC pumps are much more prone to problems (brushes / controllers etc) than an AC pump. Some A/Craft have an automatic selection here, but maybe the A320 doesn't. If the Engineers think it's a "Good Idea" to have the AC pump on - let 'em. They know more about the systems than pilots ever will and besides that - once you have shut it down - it's their aircraft. And further more they have to change the pumps as and when they fail.
MG.