I've seen this on many streams. Are they just running macros? Hopefully others have noticed this and I'm not hallucinating...
Getting warmed up, fingers moving, blood flowing etc. I thought it was crazy at first too but after playing over 600 games it really does help and just becomes part of the procedure early game.
Raising your APM. Mostly at the pro level, you will be thought less of if your overall APM is low at the end of the game. Its strange but thats just the way it is. Spamming at the beginning of the game just helps to artificially raise it.