It is normal, at least in modern guitars. As for why it's done, that's hard to say. Pleading tradition only kicks the can down the road: you're still left with the questions of why it was done in the first place, and why it persisted. One maker I know who has looked into this more or less systematically has said that tapering the body depth tends to produce a more 'even' sound. The usual reasoning goes that it reduces the chance of a prominant 'standing wave' resonant frequency that could arise between a parallel top and back. That makes some sense, but i would not regard it as 'proven'. There is also an ergonomic argument, that a tapered body is more comfortable to hold. Since some makers only taper the box by 6-8mm it hardly seems to be a major benefit, but sometimes even small changes can be significant.