Hello commanders.
In case players doesn't want units pop up in a random barracks or factory.
If we can set a specific building as primary, then all corresponding type of units will be ready right there, while still enjoying the production boost from multiple identical type of buildings.
This had been used in early C&C games, and now there are also some RTS games still using this way for specifying sites of unit production.
You can help by EXPANDING IT.