Two of the most important skills to performing music involve hearing and reading. Musicians should focus on both, however developing the ear will outweigh a lot of aspects involved with performance practice.
In my own practice, my ear is the most invaluable tool for performing, composing, teaching and researching. No matter the genre of music I perform, I need to use my ear for many reasons. Usually, I need to know and figure out the tonal center on-the-spot if I accompany singers—anyone else accompany in church? I also need to hear the balance between instruments. I need to hear my way through chord changes as well as hear how to compliment other soloists (much of what I do is improvisation grounded by song form). Most importantly, I need to use my ear to hear time and space in order to fit inside the pocket and groove. Also, I need to use my aural skills for rehearsals, so every musical aspect will line up correctly.
When I compose, teach and research, I need to immediately identify and transcribe ideas. Even transcriptions published by Hal Leonard need revision. I’d rather not waste my time solely relying on these—though I sometimes will glance at them for a second opinion.
I’m a big proponent of memorizing the sound of note names; developing perfect pitch or what I like to call, “relative perfect pitch”. It is also imperative to focus the spacing between two notes (intervals) in all ranges and on all instruments (timbres).
So what if you weren’t born with perfect pitch? You can always develop this essential skill. All it takes is patience, willpower and consistent practice each day. Moreover once you develop the ear, you will experience much more freedom in music.