Well said, Peregrina - and Darowyn.

Let's also keep in mind that we often hear a song once or twice -- on radio or in a live setting -- and are interested in learning the song before we even know its origins or who recorded it on what CD. We don't have a recording to listen to till we figure out the chords. (Plus, I think a lot of musicians, including may folkies, use modified chords and/or quite a few more than the simple 1 -4 - 5.)   In fact, unless we have the basic chords to keep us on track, if we try to play or sing the melody we just heard, it's likely to morph into other tunes that we're more familiar with. This happens a lot with the folk process.

If you can get hold of the lyrics and the chords to a song, your own performance of it is more likely to stay true to the 'original' -- at least as true as you would like it to be.   And, as I said, finding a commercial recording of or sheet music to the song you just heard can be difficult and time-consuming -- even if you happen to be affluent enough to buy a $15 CD for every song you want to learn.