Here where I live (Madison County, Iniana) there are a few RV owners that keep their TT and 5er pakred on their properties. Some beside barns and garages, some in driveways. I've noticed many of them keep their slides out all year round. Space is not an issue on your own property. So, my second guess would be, they keep them out for easy access.
If you think about it, in a storage facility, the campers are not plugged in. That means everything must run on their house battery. Even with cut-off switches for the battery, sometimes there is still parasitic draw off the battery. So, the camper is in storage a couple weeks, the slides are "in", and the owner want to get inside for something. Unfortunately, far too many campers are not designed to be functional with the slides "in". So they push the button to extend the slides ... and ... dead battery! By leaving the slides out, they don't need to depend on the battery to open everything up. It's already open. (More than likely, the owners of those RV have removed the batteries from their tongues, storage areas, whatever, and secured them against theft (maybe even taking them home). So now slides are out .... yes ... easier access.
These farmers around where I live, it has to be for easy access, I can also see they are plugged into shore power 24x7 also. (just trying to figure out a logical reason to help the OP out with his question).
If you ever have the opportunity to meet anyone parked there, be sure to ask them why. Then come back here and post their reasons. You've got my curiosity going now also!