I've asked this question to a few of my Thai friends and got the following answers:
1) So that you can get away quickly.

2) It's easier because you can look in the mirrors and see exactly how far away you are from the other parked cars.

3) So I can practice my driving skills. I feel really good when I manage to do it. (How does he feel when he doesn't quite manage it...?)

Every time we go to Makro or Tescos we have to wait for someone going backwards, forwards, backwards, forwards, and finally backwards into a parking spot. Why? It's far easier to reverse into the wider isle, than try to squeeze backwards between two cars.
Why do most Thai ladies flush the toilet as soon as they walk into the cubicle?
I haven't plucked up the courage to ask this one, so I'll leave it to those of you who are married to Thais.
VS