In my case, a couple reasons. For one, I added a switch and a couple plugs to a room so that when I turn on the overhead light, it turns on the lamps in the room as well.
For the other, I made it so the outside light turns on when it detects a family member's phone arriving home. The light is not in a place that a typical motion control would work, plus wildlife set off the motion controls all the time so they are pretty useless to me.