The above posts are substantially correct LG. It has actually been going on for years, mostly for P&O and Cunard, and the destinations may vary. The major charter airlines tend to use their traditional winter surplus capacity (as you deduced) to secure such contracts and keep the aircraft (and crews!) earning their living. Also included can be Hadj ops and, once upon a time, Downunder (wistful sigh. . . .).
It can also add a pleasant bit of variety for the few who get to fly one or more of these ops. Hope this helps.