Why would you use mixin?

You use a mixin to force a function to accept an argument at runtime even if the argument wasn’t included in the function’s definition.
You use a mixin to allow a decorator to accept keyword arguments.
You use a mixin to make sure that a class’s attributes and methods don’t interfere with global variables and functions.
If you have many classes that all need to have the same functionality, you’d use a mixin to define that functionality.