My little forum on check engine light
Chains hire only licensed and certified mechanics. On top of that, having more mechanics at the shop means having more experience on hand to puzzle out obscure problems, and means it is more likely that you will get the services of someone who specializes in your car's type of problem. Convenience is the third reason. An independent mechanic's warranty on his car repair is usually good only at his own shop. If you're caught away from home, what good is a warranty for a shop a hundred miles away? National chains, on the other hand, have branches across the country, so you're almost guaranteed to be within a short drive or tow of a mechanic's shop where your warranty is good.