Depends on the individual. In countries where the profession is legal, you see a large number of them getting into it for supplemental income. They could have a very important primary job, and they do this as a side job. Sometimes they do it for the money, which they may need to pay for a large purchase, and after that they stop. Other times, they are simply lonely, enjoy the company of their clients, and enjoy sex, so they figure they may as well get paid for something they would be doing anyways.