Xjyuk

When watching live-streams from high-level players I noticed they tend to constantly flick through control groups very fast. I watched a stream of a player switching between drone, overlord, hatch very quickly from the very beginning of the game.

That made me wonder why he would do such a thing? I sometimes do some switching between control groups myself, but that's mainly to check status of things, eg. is that chrono-boost ready?, zealot done yet?, time to inject larva? etc. But constantly checking a drone or overlord just doesn't seem to make much sense to me.

Am I missing something important here?

Most pro players do it to "warm up" their fingers early in the game. That way, when they need to produce tons of meaningful actions in the mid-to-late game, they are "limber".

Whether or not that helps or is the best method for warming up, however, is questionable. Tzenes, our resident SC2 guru, addressed this in an answer to an earlier question on improving your APM:

Epinephrine has some interesting effects on the human body, one of which is to contract the blood vessels in your fingers right before a tournament reducing circulation and making them feel 'cold.' Spamming keys is less effective at over coming this than taking a second and flexing your fingers.

A lot of amateur players will spam actions to get a higher "APM" number. As has been said countless times, though, it's not how many actions you have per minute but what you do with your actions that is the difference between victory and defeat.

You are not missing something important (all your points about checking to see if something is done is correct), players are simply keeping their fingers warm so that when it comes time to fight, they are ready to react quickly.

Day[9] does a good explanation of how tapping through control groups helps your overall game play in these two videos:

http://day9tv.blip.tv/file/4742049/

http://day9tv.blip.tv/file/4772601/

As you continue playing, these slower repetitive motions becomes quicker and quicker which to the rest of us seems insanely fast.

throughout a match, you need to regularly switch focus from your army back to your bases for production tasks. Some players use a pattern of hotkeys to help keep their base management consistent. A zerg pattern might look like this - jump to main army, jump to queen for creep colony, jump to hatchery for unit building, jump back to main army.

So, they set the hotkeys at the beginning and switch through the patterns repeatedly at the game outset, to help keep their pace consistent throughout the game. Spamming through ctrl groups at the beginning isnt merely to warm up the fingers or get the APM up.

Of course, if you are a REALLY cool pro (Like GSL Season 2 champion Nestea), when your opponent is spamming keys for no good reason at the start, you just sit with your hands nicely folded in your lap.

Its part of how they slow your production, in the code of the game extra resources etc are allocated by manipulating the gui.