I think this is, at best, a partial answer, but a partial answer to a good question: Why are movies today so long? And I'd refine the question a bit: Why do so many movies nowadays have scenes that seem to have no purpose?
Two conjectures that came to my mind don't seem to hold up: 1) the extraneous scenes are there to be cut when the movie shows on commercial TV and 2) the scenes are there to pad the movie to some minimum running time deemed respectable for exhibiting in theaters. The reason I don't think they work is because both would have applied for quite some time while the lengthening of movies seems to be a recent phenomenon.
"More research is needed."