There are all kinds of reason to have the tthing constantly recording stuff. For example, I have my Replay set to record 10 episodes of the Daily Show with Jon Stewart. It works on a first-in first-out basis. Sure, I hardly see any of them, but every now and then I'm doing something that doesn't require a lot of concentration, so I'll turn on the Daily show and maybe watch three or four episodes back-to-back. But I want to be watching current stuff, of course.
Or, for example, I watched the whole first season of The West Wing and loved it. I'd like to watch more, but I'm just too damned busy since this semester started. In this case I want it to keep all the old episodes so that when I do have time I don't have to suddenly pick up in the 6th or 7th season or whatever is currently airing.
...and so on.