I’m just curious what some of the odd reasons for a presence sensor are?
My oddest use case is I have one out in the barn watching a perch in the rafters used by an American Kestrel when they migrate thru the area and I have HA alerting me to movement when they start using it in the late fall. It’s a reminder to leave the barn door open so they can come and go as they please.
so all you need now is an automatic door opener so you don’t have to walk to the barn every fall 
I’ve thought about using a gate opener…maybe a project for another day!
I guess it isn’t that odd, but I have one in our nursery. Right now its use case is pretty basic and just sets the appropriate scene based on the time of day and whether the room is occupied or not. But I would like to create some zone based automation such as turning on a light over the changing table when you’re standing there or dimming the lights in the room when you’re in the rocking chair etc.
My daughter isn’t a fan of bad weather so I have a presence sensor detect her location when storm warnings are in the area. If she’s in the house all of the smart lights come on to full brightness. I also set up an automation on the Alexas and Siris so that when someone says ‘The coast is clear’ everything reverts back to normal.