zembla, overall it's more for exposure
I dont know the details, but sometimes when a model is new and starting out, and a photographer or editor loves them enough, they are going to want the model to become big. By doing this, they make the demand for her to be greater. For example, when Jessica Stam was new, Steven Meisel had a contract with her that did not allow her to shoot with any photographer, and someone else mentioned he controlled the jobs she would get, this set Jessica Stam into the model she is now, where if Steven stopped shooting her, it woudlnt matter because her name is already important. Du Juan, had a contract with Patrick Demarchelier and Carine Roitfeld for French Vogue, that's why she appeared in Demarchelier and Roitfeld styled editorials in the beginning of her career.