Many homes have a bunch of lights with their own switches, like lamps. Also there are rooms with multiple entrances, like a living room with a bedroom on the other side from the from the front door entrance, which would involve walking to the side of the room with the switch then walking back through a dark room after you turn it off. Being able to just get into bed and say "Alexa, turn off all of the lights" is way more convenient than checking 14 light switches around my home.