As sundayhero says, PVA gets used as an outer coat for soft 1/72 scale plastics. The reason for this is flexibility. It also gets used as an undercoat in lieu of primer, for the same reason. I've seen painters describe using these techniques on the yahoo group DBLCHM, which is dedicated to 1/72 scale plastic. Nowdays there is Rustoleum plastic primer, and many of the 1/72 scale plastic painters seem to be using some kind of floor wax as an outer coat. But PVA is a viable option.
No need to use PVA (or floor wax) on a metal or hard rigid plastic figure. Varnish is fine for them.