From a photography business owner's perspective, the reasons I use a pro lab are:

1. Paper selection - My lab offers me literally a dozen options for paper.
2. Size Matters - Try getting a supermarket to print you a 30x60. ;-) Seriously, though... I rarely print anything smaller than an 8x10, and most of the time bigger than that... Supermarkets are designed to spit out 5x7s or 4x6s by the hundreds, but anything bigger than that quickly gets expensive and slow... Or the answer becomes 'we can't do that.'
3. Eyes on the Print - At my lab a human reviews the images before they're printed on state-of-the-art color calibrated gear, and they're trained, competent and I trust them to make adjustments on the fly if need-be. Additionally I can tell them to print 'as-is' (e.g. don't touch or adjust anything!) on a print-by-print basis. At the supermarket whether I like it or not a computer algorithm is going to decide if my prints are too dark or too light and then monkey with the exposure in a formulaic way blowing out all my highlights, or muddying my shadows, without caring a bit for the artistry or intent of the pictures. And if anyone is looking at them, it's generally going to be a pimply-faced minimum-wager with no training to speak of.
4. What else can you print on? - Canvas wraps, albums, giclee prints. Try getting those printed at the supermarket.
5. When Quality Counts - Better quality ink. Better quality paper. Better quality in general.
6. Brand that Sucker! - At my lab I get a clearly printed copyright statement on the back of every print, and a gold-foil logo on the bottom right corner on the front.

Now I do realize that some of the above only apply to business owners, but in general even my students can see the benefit of #1, #2, #3, and #5 even for their personal projects and work that they just want to hang in their own homes...