Thanks Richard. No, not all can learn by ear. Even after 60-odd years I can only do it if the tune is familiar, otherwise I need the dots. To learn a tune I normally write it down in notation and play it over and over, then take the dots away and find I can't remember it! In a session I can play along generally with whatever is played, but often I am "sight-hearing", in other words I play a note as soon as I hear it played by the others.

I don't see why some have referred to needing a computer to use ABC. As has been said, you can write a tune down in abc on a piece of paper, and then play it on an instrument. No pc involved.
You can give the piece of paper to someone else to play. No pc involved.

The pc comes in either for dissemination over the web; or for transcribing into notation - although that's fairly easily done by hand, or to another format, midi, sib, nwc etc, which the pc makes very much easier; but you will only need to do this if you have a pc or synth to start with.