:lol: Let's try to make this as simple as possible. Were playing a campfire song and it only has three chords. We haven't practiced it, but were "playing by ear." The song is in the key of D. We change to the IV (G) chord. Why did we change to G? The melody (singing) didn't necessarily have a G in it when we changed. Therefore, it has to be the intervals of the melody compared to the chord. Is that what were hearing? For example, a B melody note would sound different over a G chord than over a D chord.
You changed to G for either of two reasons.
1. The songwriter already decided that he or she wants a chord IV at that point and you're following his or her directions from a song book or memory
OR
2. (if you're finding the chords by yourself) The melody notes (or harmonic rhythm) suggest that a chord change at that point is desirable and chord IV is the one that offers the strongest matching harmony as well as conforming strongly to the key. As you say, your melody may not have a G at that point but you can be pretty sure (it being a highly predictable campfire song) that most or all of the notes that it does have will be found in the G chord (G B D)
