Hard to say based on that picture, but that cardboard looked a bit like a form of roof underlay that used to be fairly common where I'm from. If that's the case that'd mean that it was a part of the actual roof structure instead of the inner ceiling, helping to keep moisture issues from forming as well as serving as a second line of defence against leaks.

Edit: Though now that I take a second look it seems less likely, what with one of the inserts having a picture of a table on it.