“…most musicians expect to perform a number of musical tasks that might seem extremely daunting to the layman: transposing chords and chord progressions, improvisation/soloing, arranging, re-mixing, navigating alternate tunings (guitar), analyzing the harmony of a song they love, re-harmonizing a song, and the list goes on. Knowing what chords are diatonic and what chords are non-diatonic gives musicians a huge boost in thinking about and approaching these tasks. Most musicians already know the diatonic triads in every major key, if not by simple memorization, then probably by rote, over time, which can help when trying to learn to play a basic chord progression by ear.”View my full answer and follow me on Quora.