This has to do with how Go handles dynamic typing via interfaces and how structs fit into that.

When a function takes an interface type as an argument, and you pass a pure struct to it, it creates a wrapper around it (this is the allocation referred to in your quote) which is simply a pair of pointers, one being a type/vtable pointer and the other being a pointer to the struct's data.

Doing it this way allows code to do run-time type inference as well as allow interface extension implicitly (that is, implementing an interface is done by implementing its methods, and you don't have to explicitly type it like ByteReader extends Reader"). Since you only pay this cost if you use it, a lot of fast-path code will use structs exclusively if it can.