I agree with Carl and forgot to mention it myself. The downward body movement one creates by stamping is yet another increase of speed/spin on the serve (when desired).
If you think of it, it's quite logical. For a good service, you use:
- your wrist
- your elbow
- your shoulder
- your trunk (twisting from the hips)
Let's say you use all these bodyparts excellently, then there's one more thing left to perfect the whole thing: your legs.
I feel it also provides me with consistency, since you kind of bend your knee to a certain extent, and then block the angle at comfortable position, hence the stamp ensures you have your point of contact always at exactly the same height.
This last sentence might seem complicated, so I'll try to elaborate a little bit more:
Let's say you've found the ideal height just above the table to make contact with the ball when you serve...
---> If you have any "bending" legs, you'll have to focus on bending them to the ideal position every time again.
---> If you block the bending angle and work with a "stamp", you automatically have the ideal position and you don't have to take it into account anymore.
These are My Thoughts on the subject, feel free to criticize