What are the use(s) for tags in Go?

In the Go Language Specification, it mentions a brief overview of tags:

A field declaration may be followed by an optional string literal tag,
which becomes an attribute for all the fields in the corresponding
field declaration. The tags are made visible through a reflection
interface but are otherwise ignored.

// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
  microsec  uint64 "field 1"
  serverIP6 uint64 "field 2"
  process   string "field 3"
}

This is a very short explanation IMO, and I was wondering if anyone could provide me with what use these tags would be?