I guess it depends what you think they mean.
I take it as "we don't know each other, I'm setting the tone as a formal and mutually respectful interaction." In my mind, the formal part is important, it's saying that we're filling roles in this interaction and that sets bounds. We're in for a financial/business transaction, so let's be on our best behavior.
At one point it bugged me (for the reasons you suggest) but now I guess I've accepted the idea of interactions that are limited in scope.
If a server/waiter calls me sir, we're talking about the meal, not shooting the shit. If I call someone sir/madam, I'm paying for their professional opinion, and I'll do my best to be a cooperative client.