It gives you insight into the moods that are coming into the room. You can check on the ones who always seem to forget something in the backpack.
It is much more than a handshake saying a cheerful good morning...it is 30 seconds that tells each of your students you care about them. You are there for them not the busyness of the morning. It is a solid spot for the students to know where they can find you and ask you questions if needed because you are not busy getting ready.
Take the time...plaster the smile on...and greet.