Ewan,
I turned 52 this year. I have been singing and playing "folk music" since I was 16 years old. If it was possible you could say I've forgotten more than I'll ever know. My memory for tunes seems to last better than my memory for words (or perhaps I just don't notice that I have lost the tune). Consequently, when my memory spits out an old tune it often is not whole. Then I punch up the DT or the Forum.
Most tunes I learn for the first time are from recordings, and I use MMario's "osmosis" method. However, if I have some time pressure (to learn a tune for a particular performance) I like the words on paper.
Pete M., my, my, my. There are times when what I think is the correct lyric is far from what the song intended. Although there is some joy in the "folk process", when singing in a group I prefer we all sing the same words (must be my German background). So a written version is helpful. I know others are "agin it".
Most every song I perform regularly has gone through a "folk process" of my own that often results in a song that is not reminiscent of the original. Then I am proud to have put "my stamp" on it. It is in some way "my song" then as well as writer's.
In my early years, when I was comfortable with neither my guitar or voice, it was quite comforting for someone to tell me what the "words and chords" were. As I gained in experienced, I found out that I often disagreed with someone else's idea of the "words and chords" and struck out on my own. I suspect those who request lyrics despite owning the recording may be in an early stage as well and are searching for certainty where none may be. They, too, will grow and become more independent minded.
Roger in Baltimore