My guess is that every AI unit always has a command assigned to it. The purpose of ForceComplete is to replace the previous command, but it also works even if the unit did not have anything to do before. ForceComplete could also be seen as "do nothing".
In addition, each AI unit has some "side activities" that it can do while assigned an other command. When it reverts to that side activity depends on the priority of the command it is assigned to perform.
If that command has priority "low", then the unit will take every opportunity to do its side activity. When the command is high, it will never perform the side activity.
The side activity of land lights is to be switched on (during night-time). Assign it a ForceComplete low, and it will switch itself on. Assign it a ForceComplete high and it will be fully dedicated to its command, which is to do nothing.
Same thing with artillery units. Their side activity is to shoot at anything they can shoot at. Tell them to do nothing with a low priority, and they will shoot at anything they can shoot at.
I use this with AA units simply for clarity issues in the editor. I don't like all these circles associated to check zones, attack orders, check points, complex triggers... It becomes messy very quickly. These force complete orders on AA get them to do their job, and keeps the mission map in the editor a little clearer.