I was looking at some fashion illustration books in the library while thinking about what Allison said to me about the difference between what an illustration and photo can do. I looked through several books showing illustrations that have appeared in vogue, paying special attention to any illustrations that appear in issues after photography was becoming the norm. I came to the conclusion that due to being able to stylise drawings more than photos (especially in the early days of photography) means that illustrations were used less to give an accurate representation of what clothing looked like and are more about presenting an attitude, lifestyle or idea. The complete freedom of illustration (as compared to photography, especially back then) allows certain amounts of abstraction, stylisation and freedom of background to suggest more abstract concepts or attitudes that the magazine wants to convey.
Another reason why Vogue specifically used to include so much illustration in there magazine and had a tradition of illustrated covers for a while even after photography became a valid option is initially Vogue wanted to present fashion more as an art than anything else, to bring a sense of sophistication or class to itself and the merchandise it was presenting. Illustration more easily provoked these ideas in people than photos.