I get it now.

When I worked at the diner, we tipped out the busboys on our tips. We didn't make money, they didn't make money.
I got the concept of tipping on sales, but I didn't realize what a dent it actually made in your pocket if you're making big sales with small tips - or worse, stiffed. That's what people mean when they say they have to pay to wait on you when you don't tip.
My first table today, very pleasant, took my drink recommendations, smiled, said please and thank you. Paid with a credit card, crossed out the tip line, nothing on the table. I was really upset, I couldn't think of what I did wrong. I eventually figured.. I did my best, they might have just been looking for a reason not to leave a tip. Then I got a 10% tip, they were obviously just cheap.
But what really got my goat was the couple with the coupon. They got the complimentary app, he only ordered soup and salad, and she ordered the most expensive seafood dinner. The coupon took off her entire entree, about 20 dollars. Of course, he tipped on the discounted amount like I hadn't been refilling their iced teas six times each. That was just depressing.

Anyway, not that I would have ever tipped any less than I do (generously, obviously, I know the game!) but I didn't realize the effect this different way would have on my own pocket. PLEASE make sure you always tip before any discounts or coupons. Just.. please tip in general.