If I remember correctly (from other games), control groups means that you can select a group of units and assign a number to them (Ctrl+n?) so you can then issue commands to them as a group.

I think that a human player's game would be significantly slowed down if they had to play without control groups. Accordingly, the ability to control units without having to assign them to control groups must be a very big advantage for the purpose of micromanaging units. It would be like playing a turn-based game when everyone else was playing in real time.

True, you might be right. It might be using an abstract notion of control groups internally (learned through watching and playing so many games), capped at the typical control group cap, in which case maybe it's not an advantage. But maybe it is able to have much more fine-grained control over every single unit, in which case it probably is an advantage.

It was also selecting buildings that for a normal player would either be out of the isometric field of view, covered by ui elements, or only barely on screen like clicking 1 pixel specific in the corner.