I learned piano first and always thought of songs in terms of melody. After learning to play guitar (very little), I thought of songs as chord progressions. For me it's easier to write a melody (or harmony or descant) when singing along with a guitar and hearing the chord progressions than just trying to pick notes out of mid-air with no underlying structure.

Several months ago I was in a singing workshop led by a blind couple...let me see if I can explain what they said...

They were trying to convey (in layman's terms) how to sing harmony. It seemed to me that they were talking "chords"...but they emphasized that sometimes the first note of a song is not the root (bottom note in triad form) of the chord that harmonizes it on the guitar. (Also it's not always the first note of the tonic scale, they said doh, re, mi) Anyway, they would play the chords to a song, find the melody note (which may or may not be in the same position in the chords they played), then they would sing other notes from the chord in order to harmonize. I think that when someone is very familiar with hearing guitar chords they can find the harmonizing parts easier.