There are a couple of possibilities. Firstly, the headset could be a standard threadless one, in which case that clamp (which may in fact be a seatpost clamp, they're available in the right size) would simply allow the stem to be removed without causing the headset bearings to go out of adjustment. I suppose it might make sense to set a bike up that way if you weren't sure exactly what length stem you wanted, as it would make it less of a hassle to change out stems until you found one that fitted, but unless you were doing that, it wouldn't really make a difference. The only time things like that really turn up are when they also include a cable stop for a cantilever or centrepull front brake, which that one doesn't seem to, so it's a bit of a mystery why it's there.

The second possibility is that your headset is similar to some of the earlier threadless designs, which had a clamp-on fitting on the fork steerer that the upper bearing cone threaded into, allowing the bearing to be adjusted to remove play in the same way that the star-nut, cap bolt, spacers and stem do on on a conventional threadless system. As far as I'm aware, headsets like that haven't been made in quite a long time, so the first possibility is more likely.

Those are the only two things I can think of, and either way, it's not going to do you any harm, so you may as well keep it there.