If you play Zerg,this process is far more important than other races.Because the larvas and the units are mixing each other which may create control problems according to larva’s stable characteristics.The rally point brings order at playing especially multiplayer games,if you are making micro at any race the rally point importance  becomes more important because the unit’s seperation is very critical if you want to send them to the warfare zone properly.In the older versions of Starcraft the Rally ability is only can be set by using Rally point button or its hotkey but in newer versions the Rally process can be done by just right clicking to the wanted location even when the building is in construction mode.You must construct your buildings dependent to your rally positions and production priority to make the game easy.