Some people just do it because they don't plan on doing it for long and just want to have extra money, but there are lots that need it to survive because the average wage is barely enough to feed yourself let alone get housing. Wages are around $1000-$1300/month and rent is usually between $1000 and $2000 depending. With studio apartments going for closer to $1200.

Only way to live is with roommates or some financial assistance, or you work 2 jobs. One for your rent and another for food.