I think the simpliest answer to your question without elaborating into different genres, paid jobs, test shooting etc, is just to say, that people pay models to use to practice their lighting, posing, communication and direction skills etc.
I've never paid a model for a test shoot (read portfolio shoot), I test in return for my services, the better your portfolio, the better models you will get approaching you for "test shoots". I'm fortunate enough to test for several model agencies, so my access is good. It's like a hierarchy, and when you're beginning you have no shots to show, so MAY have to pay for the pleasure of using a model.
I dont shoot glamour, but it all works the same way, you'll never learn unless you just do it.