The terran mech build needs to rally all factories to the top of his ramp and then his natural early game. If his army does really well and he has too much money he needs to build more factories at new bases and add factories to the hotkey control group.
There comes a point in the game when my workers have left the first three bases and I prefer to have my next army spawn where the workers are. And having all the control group barracks or factories rally to one point causes units to move across the map on non-attack move. So I would like to cancel all rally points for my factories and have units pile up at the doors they came from. Then I can just lift them up, add a reactor and fly them to the corners of the map.