I think its is the amount of residue water left in the van.
With ply boarding the wood tends to soak and if not drying out will eventually rot.
Same with a van floor, if water is under the tank fixings it may eventually rot the metal.
I had a van for 2 years constantly soaking with water being sploshed around and when I got rid the floor was rotten!
Personally, I treat every vehicle now with a protective coating.
regards,
Trev