Ah, Mr Red, you've hit on a particular itch in my undies, sticking doors. When external doors are hung, by chippies, they are hung as manufactured, primed/varnished on both faces but NOT painted on the edges. And they are usually hung with fixed-pin hinges so that they are not readily removable.
This means that when the house, along with the door, is painted, the bottom edge is, and remains, unprotected wood. Which swells and drags on the sill, stressing the door all Winter.

I used to maintain a factory staffed by people who SLAPPED their way through the doors which had lever-style doorhandles, destroying the stainless-steel latches in about four years. Time after time.
It saved them from breaking step on their way through.