Well, threshold are important; they got a lot of symbolic oomph behind them, even on their own and just... naturally, y'know? It's why, in so many stories, some things---like vampires!---can't come in without an invitation. That stuff above the door kind of amplifies, kind of channels that oomph, though some bits of it also make our claim on the living space more... Concrete. Concrete, official, backed, because it's been put down. It's set that you and I are the only ones with the authority to issue invitations.