they control there groups, for example #1melee units #2 range #3some buildings etc...in the beginning this is just spam, they set 10000times new waypoints for buildings etc...but keep this high apm from the start helps u in fights, u can control (micro) ur units better, and do more things at the same time (macro), for example build a new unit right after u lost one, or build other stuff while u in fight.
But a high apm is not all, see Axlav(dont know if this is spelled right^^) hes a very good player and got ~100-150 apm, the sickest player nowdays is FoCuS imo, always got 350apm+^^