I saw Paul Bassett use that trick in the Coffee Crazy DVD when using the Sunbeam machines.
I started doing that on the breville machine and still do it on the Boema commercial machine.... out of habit now I guess.
Wetting the coffee I assume expands it to take up the small air gaps between the shower-head and the tamped coffee.
Nothing wrong with the shots that I pull so I tend to continue to do the same thing! ;D