From a business stand point, there is very low start-up cost. A model is renting out thier body for a certain amount of time. In some cases its just for a private show. When Ana Wintour (Editor of Vogue) goes to a showroom to view a new collection before its released publicly, the models are there just for her viewing perhaps just for 2 hours of waiting time, fitting time, makeup, more waiting, and a few walk-throughs. If it is an internation campaing for Loreal, then the payment is much higher, but you will be leasing the rights to use the images of the face or body. In comparison with say a car rental business, the profit margin can be extremely higher.Now, having said that to kind of squash the common notion of vanity, there are those who want the glitz and glamour that they think Gigi Hadid has. They want to have that public feedback that they are beautiful. They dont realize that modeling is a stressful job and not just as easy as getting your picture taken. There is a misconception that if someone becomes a model, actor, or singer that they will make lots of money and be famous. (Nevermind that Gigi Hadid and her BFF Taylor swift already come from money just like the Kardashians.)

Source: Why do some people want to become a model? – Quora