I can't speak for bigsallysmom's reason, but my own would be stealth/security for in the city once parked, but also with the freedom to remove/slide door and passenger 'side' out of view when driving or boondocking. (my van is cargo with window in sliding door...much safer for lane-changing with that visibility)
My build plans are to attempt something similar to Dgorila1's sliding door suggestion. My van Moonfreed is a Ford with the seatbelts mounted way back requiring a little bit of an 'angled' wall next to the passenger side of the vehicle. That 8-12 inch angled wall will be bolted down. The passenger side bulkhead will lock to that angled wall and the floor/ceiling and be sitting in the furthermost forward 'track' (like sliding closet door track). I'll have to figure out how to inset locks/latches into the door itself, since it will have to slide in front of the middle 'main' access door.
So the top-down layout would roughly be:
/
XXXXXXXXXXXX------------------¯¯¯¯¯¯¯¯¯¯¯¯
X = drivers side pocket bulkhead (main door slides in there)
- = main door center (slides into driver bulkhead)
¯ = passenger locking/sliding bulkhead (slides all the way over in front of driver bulkhead)
/ = angled 8-12" bolted passenger side wall
That's 'the plan', anyhow....sourcing the hardware may be interesting.
Hope it's of help, bigsallysmom.