Check engine light for dummies
Chains reduce the amount of time you spend waiting for a repair to be done because they can afford to have larger inventories of parts on hand, meaning you and the mechanics don't waste time waiting for parts to come in. Chains also have more employees than all but the very largest independent shops, so they have the staff on hand to a two man car repair easily and to speed up repairs by adding more mechanics if the job can go faster with more hands. The second reason is an assurance of quality. 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.