I have something on the order of 50,000 coins at this point, and there's no clear use for them. What are they for?
There are some tools available in the menu > buy icon. You can see friends progress, overall progress as well as getting better tools for cube mining. The menu icon should be on the bottom right of the screen (or menu button for android devices).
Not sure how all of this works cause I do not have enough coins to afford it and servers are pretty overcharged.