My little obsession on alternator replacement
Speed is the first reason. Chains cut back on the amount of time you have to wait for the mechanics to get necessary parts because they an afford to keep a large inventory of parts on hand. 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 make certain that all their mechanics are licensed and certified.