Over the course of an evening or lunch, waitstaff build up a far more complicated relationship with individual customers than someone at a till. If they do this well, tip them. If they don't, then don't.
A waiter or waitress looks after you, possibly for two hours or more. They use their skills to judge if you need something (above and beyond clearing your plates when you've all finished), to judge what kind of guidance you'd like with the menu and wine list, their knowledge to advise you, to judge whether you'd prefer chatty and invovled service or something more unobstrusive, hurried or unhurried. In acting as a go-between they represent you in your relationship with the chef, and vice versa. If the chefs are pissing around (theirs is a bloody stressful job so it's fair enough theat they get fiery sometimes) they stand up for you. If your pissing around they stop the chefs from going off on one (not that you see any of this).
All this and more is why waitstaff get tips and checkout staff don't. It has, imo, little to do with being on the minimum wage.