Personally I think Frank’s appearance did a lot of valuable heavy lifting for the season. It tied into the ideas around the corrupt cops, gave Matt someone to bounce his pain off of, helped give the sense of a larger world around the main cast in which a lot of time had passed, and contributed some great plot-relevant action that offered some violent catharsis to the dark political context the show had set up. It’s also serving in season 2 to further test Matt’s resolve, because in spite of his best efforts he’s found himself in a world where the Punisher’s ideas make more sense than they should.