Protocols aren’t about inheriting data fields, they expose standardized behavior across data types. Think about polymorphic methods in OO languages.

You have a Shape, Circle, Square, Triangle classes. Maybe Shape is an interface or abstract class. It defines certain methods common to the shape family of classes that each subtype must implement. For example, an area() method. Each Shape is able to compute its own area, however, those calculations differ depending on subtype.

With Protocols you can do something similar. You can define a Shape protocol along with protocol implementations for each Square, Circle, Triangle struct you might have. These protocol implementations could be defined within the given modules, or supplied externally in another module. The point is, you can have a function operate on data for which there is a Shape protocol implementation in the same way, regardless of the actual underlying contents of the data.

This is how the Enum module provides standard functionality for processing Enumerable things. Enumerable is a protocol that defines four functions. If you define a protocol implementation that implements those four functions for your data, then you can leverage the Enum module for your data as well.