Okay, I have some expertise in this particular question.
1. The defensive wall is set up to prevent a straight hard shot on goal. From 20 yards out, without a wall, anybody could score pretty easily.
2. The keeper has to stand to one side of the wall in order to see around it to know when the ball is kicked. If he stood in the center of the net, he would not be able to see the kick.
3. He stands to the far side, because the kick mus somehow go over the wall and this will take longer than for it to go straight into the net on the side that he is standing. Thus, the keeper leaves more room to the side that he will have more time to react.
4. Another possible reason is that all keepers have a strong side and a weak side. This means that there is one side that they are more comfortable diving to than the other. If possible, they will set it up so that they will dive to their strong side if neccessary.
5. In the saem fashion, most strikers have a good foot and a weak foot. By position the wall and the keeper, the defence can take advantage of this.
6. When I played professionally, I would never set up a wall more than four men wide. I think that most keepers make the mistake of putting too many men in the wall, and this limits where they can position themselves. But, then again, a good free kick can beat any wall, and any keeper no matter how well they are positioned.