>>Dangit+(OP)
The claim is that by not renting out some of your properties you can raise rent on the rest. But the way you make money from properties is by renting them, so if everyone else is refusing to do it then you get the rents from anyone who shows up.
That and if you don't rent out a property for long and leave it unused, it'll literally rot because nobody is there to notice squatters or water leaks or etc.