I'm working on a project right now that requires me to draw dozens of people doing all manner of things, spread over a number of pages. I used to think (somewhat naïvely) that illustrators could usually do such drawings without referencing anything. But in order to draw or paint realistically, the artist needs to observe reality in order to translate it.
My budget can't afford models, so the quality of my 'realism' is going to suffer a bit for it, and I'll make things up the best I can. Norman Rockwell got to the point where he could afford a photographer to gather shots of his friends and family and neighbors. Other illustrators of the past had photographs taken of their models (and sometimes themselves) in the most precarious poses, simulating the action required for their commercial assignments. Many modern illustrators utilize digital cameras to pose themselves, friends and volunteers. Most times we can't afford to actually pay someone to pose, or in many cases afford the time to take photos.
I was really taken by surprise when I found out that a number of 'realistic' cartoonists used live models to pose for the multitude of poses required for a good storyline. Milton Caniff had a great swipe file, but he also used models for key poses for Terry and the Pirates.
And then I found out that Alex Raymond used live models for Flash Gordon—but what really surprised me was that his models were nude, for poses that were fully clothed. I understand that the artist needs to understand the architecture of the body under the clothes, but that seems a bit extreme for a comic strip. Remember when Raymond's style morphed from a stylistic brushy expressionism to hard-lined realism? I do believe that's when he could afford to have models at his beck and call. The strip at that point was nice to look at, but I believe suffered artistically as he left his brushwork-feathering behind him.
Take a look at this 'run-of-the-mill' strip:

Alex Raymond — Flash Gordon — March 2, 1941

Above, Alex Raymond drawing that particular strip.
Above, the line art for one of the panels.
And above and below, the model that he required for his realism.
Above, even for this seemingly insignificant pose of Dale behind bars, he used the model, below, to act it out.
I'm going to check my schedule and budget one more time—maybe I could afford some nude models for the dozens of poses I've got ahead of me, after all.
Oh, crap, most of those poses are supposed to be of old men.