The original reason for having a minimum wage was that some employers would pay as little as they could to their employees in order to have a higher profit margin. A minimum wage puts a limit on how low wages can go. However, minimum wage is not always a living wage (especially if it's supporting more than one person). And, contrary to popular belief, most people working for minimum wage are working because they have to. The service sector is an increasingly large part of the labour market, and many people of all ages and educational backgrounds are working these dead-end jobs. And even those kids you see flipping burgers at McDonald's probably aren't there for the fun of it, or simply for spending money. Many young people work part-time (or, more often full-time, or have more than one job, especially in the summer) to put money away for school, or simply to put a roof over their heads.

Any worker 'worth' less than minimum wage is unlikely to last long at any of these often demanding jobs. Most (if not all) minimum wage workers make a lot more than $5.75 per hour for the business that profit from their labour.