In a building such as the stable it says income from horses +5 (or other special resource +something). What does that actually mean?

According to what I have read simply having the resource is sufficient to build anything, in that having two sources of horses doesn't change the cost of any building/unit. And having a resource does not provide income because they are purchased from the "global" market instead of from players. So I wonder, what is the advantage this provides?

So extra income comes from having the building not solely having the resource. That works. Thank you.