it's a form of apology, so it's not retarded, sometimes people just pop through the door not realising someone was mopping, notice the wet floor... this is also why they have those "wet floor" signs, to try and prewarn people so they don't do it, or if they can, take another path where they were going

i actually worked as a contract cleaner and occasionally i had to deal with this... mopping the floor was a fairly time intensive part of my job, almost always the last thing, so there's lots of reasons why it's impolite to not make a gesture of apology in some way