Imagine that the Red Player is using hotkeys, and the Blue Player is not.
Let us assume that Blue is adept at using her mouse, and Red is equally adept at using his hotkeys.
The most common way to use your 10 hotkeys is to divide the keys into unit groups and building groups.
Most Starcraft II players are finding that 4 unit control groups is usually sufficient; so keys are used for groups of buildings.
You can manually edit the hotkey file, and, although probably illegal for tournament play, there are a lot of fun tricks only possible this way.For instance, control groups, base centering, and camera bookmarks are not accessible via the mouse.