What are the IOS IAP?

Hit the buy button and was about to put in my PW then I saw there was IAP on this $7 game.

Does anyone know what they are and why would a premium game have IAP to begin with?

Thanks

They’re cosmetic skins that allow you to change the way your creatures look. You can obtain these skins just by playing the game anyway, though. They’re mostly there for players who want to support the game, as many people feel that $5 isn’t much to ask for a game that can last for hundreds or even thousands of hours.

Thanks for the reply. I had the first one and enjoyed it. Was just surprised when I saw IAP and unfortunately there was no list of what they were.
It’s downloading now.
Cheers