What do the coins buy in this game?

You can shop for a couple of items, but not much. For example, I mostly buy the “fan” tool (5 for $25,000) which allows you to randomly rearrange the Match 3 pieces when there are no good moves that will match the pieces I want. There are some other items you can shop for, but I never found them useful. Also, sometimes you can use a “tile” and money to open a new chapter in a book. I hope this helps.

Not all things can be purchased with game coins. It will tell you in the store if you can purchase with coins and how many. Just scroll through the items; it's easy to see.