The purpose of this library is to make it easier to modify existing code, eg to make existing classes implement new interfaces. In particular, it seems to be used for Minecraft modding.
Yes, Minecraft gets more and more data-driven, but for adding completely new features like, a solar system with planets, or turning the player into a robot of something, you'll have to edit some of the built in code.
This Mixin library is very useful, since it allows different mods to edit different parts of the same code without interfering for example.
It's also very easy to use compared to jvm bytecode manipulation, you just write a normal java function and put an annotation on it, and boom, now it runs right before line X inside of class Y, at runtime !
