When asked in an interview what was his motivation to play the flute, I recall Ian Anderson saying that he learned how to play the flute because, well, just because he wanted to learn how to play it. End of story.
Ian has said many times in interviews that he was a guitarist but felt overshadowed by the hot guitarists like eric clapton and thus picked up the flute as there was less competition. Interesting that martin barre also played flute in his band prior to jethro tull.
He learned fast and was performing on the flute within days of starting to learn. Might have even performed without a day of purchasing it. Fuzzy on the specifics. But he almost immediately switched to performing on flute along with occasionally performing on acoustic guitar on stage.
Ian can play basic keyboards but write his songs on the guitar. He didnt provide anything but basic chords - no real charts to bandmates. He taught the songs to bandmates by playing the song on the guitar . Martin and others figured out their parts.
He has said that he learned to play flute when he realized that the rock world was full of great guitar players and he would never be as good as any of them. He did it to distinguish himself.