I don't see the need of it if you already have a class-based system.
Go uses structural typing. But it is the only thing they have. Rust has traits, and it is the only thing they have.
I can't think now of more stuff but I remember you could even write xml in the language.
Like in Java?
new Object() { void foo() {}}.foo(); // Works
The point about XML is so stale already that I won't even bother commenting on it.
I care about simple, consistent rules. "X is a type, you can use it wherever you want" is one.
"X is a type, but there is also Y, which you can't really express in position A and B unlike X, but can only use it if you carefully avoid doing things #1 to #4" isn't one.
I don't think it helps for consistency when you have to ask yourself why some functionality is defined in terms of a trait if it could be defined in terms of the structural type and just providing the methods would be sufficient.