Older pet battles still have some merit for leveling pets, but award shiny pet charms. I have a silly amount of these shiny pet charms. Are the useful for anything that I might want? One of the nice things about pet battles is that pet experience can come from almost anywhere.
Being able to buy stones with shiny pet charms at 10 times the polished pet charms price would be really nice.
You really cannot do much with these anymore, shiny pet charms are used to buy garrison pets and toys, and some new dalaran pets. https://www.wowhead.com/item=116415/shiny-pet-charm#currency-for
It feels wrong to still be getting old, almost useless currency from content that is still as hard as the first day it was released (some battles are even harder after the decided to nerf various OP combos).
You can sell the pets you buy with them on the AH, can usually get a few thousand gold. Other than that, not really.
I used the last of mine to buy pets, level them, and then sell for gold on a high pop server (so I could buy other pets at lower prices). I wouldn’t try to sell them on my home server, but they did well on high pop ones.