While there was a good discussion of [using a lab vs. a personal printer](https://photo.stackexchange.com/questions/3348/lab-prints-vs-printer), I'm curious what folks see as the advantages to using a pro photo lab as compared to the local drugstore or supermarket. Why should a photographer send files to someone like Miller's or White House Custom Color instead of just having images printed at the local WalMart or Walgreen's?