GameFreak probably made tipping in the game possible for the purpose to make the game act like the real-world. Tipping probably isn't really important, and is there for the extra game decoration/dialogue of characters.
There are a few topics concerning tipping, with all of them in a rumor phase. The most famous rumor is the rumor about shinnies appearing more frequently in battle/breeding. There are a couple more topics concerning the reason of tipping.
Now from my opinion, I believe that tipping has some hidden potential. It may increase luck in general, personally. No, not shinnies appearing more frequently, but the theory that I believe which makes your luck strengthen. By luck, I mean catching good Pokemon with IVs, Pokemon holding items, rare Pokemon appearing more frequently, etc. There is really no evidence to support this theory, and I can't confirm this personally, but we never know what GameFreak planned.