Interfaces are ideal for defining mixins

A mixin is a type that a class can implement in addition to its “primary type” to declare that it provides some optional behavior. For example, Comparable is a mixin interface that allows a class to declare that its instances are ordered with respect to other mutually comparable objects. Such an interface is called a mixin because it allows the optional functionality to be “mixed in” to the type's primary functionality.
Abstract classes cannot be used to define mixins for the same reason that they can't be retrofitted onto existing classes: A class cannot have more than one parent, and there is no reasonable place in the class hierarchy to put a mixin.

Generally speaking mixins provide some additional support and they have interface which generally ends with "able". Ex. comparable, callable, iterable and so on.