This typically is just part of the commercial agreement on the wet lease. There's some additional return to Cityjet on these flights, although they may not have any real part to play. It puts them on the Cityjet booking site (given that they have one) as well as the KLM one. If Cityjet offered such flights themselves they might get just a handful of pax. Well, that's all to the good to put them on a flight KLM are running anyway.