If you declare specific error types and callers only write handlers for specific cases, then adding a new error breaks them. If you just declare a base error type in your API, they have to write a generic error handler or it doesn't type check.

In this way, declaring a type guides people to write calling code that doesn't break, provided you set it up that way. It makes things easier for the implementation to change.

Sometimes you do need handlers for specific errors, but in Go you always need to write generic error handling, too.

(A type variable can do something similar. It forces the implementation to be generic because the type isn't known, or is only partially known.)