为什么结构声明有时包含空白字段？

From the golang spec

// A struct with 6 fields.
struct {
    x, y int
    u float32
    _ float32  // padding
    A *[]int
    F func()
}

• Are there any practical scenarios of using the blank _ fields inside a struct? (some code snippets would be appreciated)

The padding is exactly what it is called: Some padding to align the following field to your needs, e.g. to match the layout of a C struct. It cannot be accessed (at least not without package unsafe).