Mmwave sensors, what do you do with them?

They seem to be the most popular sensor discussed on HA forums (not just this sub), maybe in par with leak detectors. I might be missing imagination, but I still don't see a practical usage that makes them worth the hype. I understand that you can turn on lights where people are, but I don't really have a use for that (maybe my house is not big enough), anything else of interest?