https://www.musical-u.com/learn/who-moved-the-tonic-part-1-hearing-key-changes/
If you’ve been working on playing by ear – with or without solfa – you know by now that the tonic gives you most of the information you need to play and/or transcribe a melody without written music.
It tells you the key, the scale, the accidentals, the potential chord progressions and more. And if you can pick out the subdominant (fa) and the dominant (sol) by ear then you can figure it out even faster.