I've seen "Offset door" boxcars on Conrail about 10 years ago, and I've seen a leased one with plug doors. I have photos of a couple somewhere in my collection. I always thought these were used for customers with short spaced, or unevenly spaced loading docks.
For example. Customers having large warehouses with multiple loading dock doors the length of the rail siding, would receive strings of boxcars at a time, with the docks/ramps spaced apart to match the door openings on a coupled cut of boxcars. Since the 70's the length of typical common boxcars expanded from 40' to 50', and then later to 60'. In many cases it was not cost effective to rebuild the shippers warehouse walls to respace the dock loading door centers. And plus, unlike in the past, it was no longer considered cost effective to pay the railroad to have a devoted engine and train crew to switch around the cars positions multiple times during the load/unload. Thus dock doors or ramps spaced 43' apart rather than on 53' centers could be fully utilized by including 1 or 2 offset door boxcars.
Also, I could see where the short length of a smaller customers siding would only allow the last car to be positioned such that the car's center does not line up with the last dock on the end of the warehouse building. Maybe the door on a typical boxcar, only lines up with the corner of the end of the building wall, and does not reach to the center of the last dock/ramp on the building wall.