Why does golang RGBA.RGBA() method use | and <<?

Question

In the golang color package, there is a method to get r,g,b,a values from an RGBA object:

func (c RGBA) RGBA() (r, g, b, a uint32) {
    r = uint32(c.R)
    r |= r << 8
    g = uint32(c.G)
    g |= g << 8
    b = uint32(c.B)
    b |= b << 8
    a = uint32(c.A)
    a |= a << 8
    return
}

If I were to implement this simple function, I would just write this

func (c RGBA) RGBA() (r, g, b, a uint32) {
    r = uint32(c.R)
    g = uint32(c.G)
    b = uint32(c.B)
    a = uint32(c.A)
    return
}

What's the reason r |= r << 8 is used?