Learning to play The Piano

If you play in an orchestra, you and the harpist can more easily get on public transport since someone else carries your instrument. I suspect that the most common real-world reason, by far, is that parents make their child take piano lessons. Some go on to like it and an occasional younger sibling will try the piano out themselves and actually ask for lessons. Few people set out to play a particluar instrument (with exceptions such as Bix Beiderbecke [trumpet], Mark King [electric bass]).