Regardless of the trend today to take a fly-on-the-wall/candid style photos, sometimes we as photographers actually want to portray something very specific, something we see in them, as Karsh did.
People like Churchill aren't necessarily going to do what we photographers ask, remember in his day job he was generally in charge. Some times we may even need to tick them off to get it.
As to walls (actually to all backgrounds), they have character, they tell us where the subject is and what they might be up to. I wouldn't portray say Japan's PM with the same wall Karsh chose for Churchill, it wouldn't make sense.
We can when we please apply many principles from painting. Every element of a portrait can be chosen.