结构内的功能。 为什么？

What is the use case/advantage of defining a function within a structure in go?

type demo struct {
    F func()
}

I think that the best answer would be an example.

Look at Client.CheckRedirect in the documentation.

type Client struct {
    // (...)
    CheckRedirect func(req *Request, via []*Request) error
}

This is a function that is being invoked whenever a http.Client has a redirect response. By the fact, that this function is a public property, you can set this when creating the Client object or afterwards and thus you can define custom behaviour on such case.

client := &http.Client{
    CheckRedirect: func(req *http.Request, via []*http.Request) error {
        return http.ErrUseLastResponse
    }
}

Function properties are just a delegates of custom behaviour (and not only!).

Another example would be creating an object which has an event.

type Example struct {
    EventHandler func(params []interface{})
}

You can specify a behaviour on that event by setting the Example.EventHandler property.

It allows you to customize the function for a type without making it be from that type.