A lot of languages force you to start with this from day one, unless you want to go refactor everything to use an interface later on, so people just do it even when there will literally never be a reason to (and for testability, sometimes).
The cool thing about Go is the interface system is inverted kind of like duck-typing, so if you write purely idiomatic Go, then the thing receiving an arg in a function call is the one specifying the interface it must meet, rather than the implementing code having to declare every interface that some implementation meets.
People screw this up a lot though, especially if they came from Java/C# backgrounds.