One of the main uses for interfaces in Go is defining the contact for _your_ dependencies. Rather than saying your function takes a socket, if you only ever call Write(), you can take a Writer, or define another interface that is only the set of functions you need. This is far more powerful than declaring that your type implements an interface up front. It allows for things like e.g. multiple image libraries to implement your interface without knowing it, enabling your project to use them interchangeably. And as another commenter said, you can have the compiler verify your compliance with an interface with a simple (though admittedly odd looking) declaration.