Why Is The Photographer Using This Lens?

Here is a few shots I took of the photographer in the process of taking photographs of the model wearing some clothes for the new lines and on the right I zoomed in to the show the lens he is using. It was very interesting to watch a professional photographer at work, I'm sorry I didn't pay more attention at the time and get the camera and lens details to add them here but I hope you can see enough of the lens to be able to comment. 

My question is why is the photographer using what looks like a long telephoto lens, as you can see he has to put quite a bit of distance between himself and the model, presumably this is around the minimum distance to be able to get the model into focus and I understand it these telephoto lens will also need a wide aperture in order to have sufficient light and that will flatten the object making it very hard to get subject to stand out by blurring the background. I'm absolutely sure he knows what he's doing, I'm not questioning that, its just that I always thought a modest 85-13mm lens was best at this job.

Anyway its an interesting question and has been puzzling me... So I was wondering why use the telephoto lens and what lens would you use?