What's the point of this? Why not a totally random draw?
To ensure "random" doesn't give you a couple of groups like "Group A: Argentina, Brazil, Spain, German" which would mean 2 of those teams would get eliminated, and they are the teams all people would like to see on the head-to-head closing stages. Also it mixes regions up, as you could get a random group of all-european teams which would not make it very "world"-cup..
Basically, makes the group stage more interesting.