Earlier quoted context omitted.
Also: interfaceiritus. Someone saw "accept interfaces, return structs" somewhere and now EVERYTHING accepts an interface, whether or makes sense or not. Many (sometimes even all) of these interfaces have just one implementation.
Doing this allows you to mock out that implementation in unit tests.
> Many (sometimes even all) of these interfaces have just one implementation.
They are missing that mocks are the second implementation. (It took me years to see this point.) I would say that in most of my code at work, 95+% of my interfaces only have a single implementation for the production code, but any/all of them can have a second implementation when mocking for unit tests.