Well, it's pretty obviously that locking a door prevents going into that room, the point is how does that help alleviate the issues caused by short term rentals?

Buy two units sharing wall. Add a door. Now call it single unit. Keep door in between locked and rent the whole thing. You can even multiply this multiple times over.

Made to avoid that sort of loophole exploitation.

It makes it hard for the renters to have a separate space.