It's to help manage all the emails when he returns, only some of which will still be relevant at that point.
Consider the case where Dave's response causes the sender to realize, "Oh, well, I'll take care of the issue this other way then."
That's one fewer email Dave will have to deal with when he returns. The traditional out-of-office strategy would result in the initial email, and possibly even a second "Oh, don't worry about it, I already took care of it." followup email.