The ones I have seen have been in hotels and on cruise ships. They give the illusion of a folded up comforter or quilt. They hung down below the mattress on the sides and were maybe big enough to cover the pillows if they were at that end of the bed. I am sure they save the maids a lot of time as they change the sheets everyday and wouldn't have to struggle with the comforter that way. The ones I saw were not pieced and had no batting they were just a lined strip that went acros the bottom of the bed.
There were two ladies on an episode of Simply Quilts several years ago who made quilted pieces that covered the pillows and paired with a plain bedspread looked very nice and could be changed for different season. I believe they were similiar to the runners that were popular for mantles.