Be able to put a sequence of “building flags” (aka gather/rally points), for the newly trained units to follow.
The tactical reason:
Some times you need your reinforcements to take a detour, either because 1) an enemy Castle is on the way, or 2) because the enemy sees the reinforcements line with the LoS of some random building of his, inviting him to start picking them off.
It’s a broken feature since it uses the Shift key, I will make a sepeate thread about that.
Possibly not by bad intention, but it doesnt work since the hotkey collides.