I agree with much that has been written here on transposition. I would like to add my view.

With a background of Guitar and chords I rely on the left hand to guide the band and enhance the melody. (Right hand) I understand that many organ players follow the bass pedals and allow the bass line to select the proper chords.

My school of thought is that most people can hum or whistle a melody so I do not give that much thought. The more you just kinda search for the melody the more proficient you become. After a while you just think the note and find it on the keyboard with no other thoughts present.

Not so with chords, (Harmony) Your left hand is telling the keyboard where to go. It is also flavoring the harmony to add color to the song. Five foot two can be played a two-step or a jazz tune and it is not just the style that you change --you also select chords that flavor the type of music/style you are playing.

Having said that let me go back to transposing. Normally the repeat bar or bars at the end of a song gives a perfect introduction to the song in the key that it is written. Change that repeat bar to the new key and you are now modulating to the new key. Do we have anyone here who have no clue how to transpose?

Transposing puts the song in the correct range so that a singer is comfortable.
Transposing puts a solo instrument in a key, which enhances his instrument.
Transposing takes the boredom from the song. it adds interest. The song Mack the Knife would lose a great deal without transposing. (Modulating)

Any of this help, I’m just shooting darts.

Playing by ear is the end, it is where music loses technology and gains feeling. It expresses much of what we are. After a year or two it is fun seeing how have changed the song from the original. Aaaaah!

John C.