There is a lot that is difficult about classical ballet. Musicality in ballet class training makes it easier. What exactly musicality is, can lead to lengthy discussions about music, personality, and more.
I once had a teacher who described it as "technically dishonest" to use or feel the music in any way in order to perform a step better.
You mean we should have been able to train with someone just hitting a pencil on a pot lid to keep us working together? Or have a metronome clicking in the studio?
This same teacher made sure that her pianist played a battement tendu type of music for jumps - a little plonky, a little pedestrian (though there were pianists who, luckily, could not play without feeling). It seemed that inspiration was a way to cheat.
There are dancers who will catch your eye in class, even when they are not the best technical student, and are not among the most physically gifted ones. Their musicality is more than being on the music. Some people describe it as being "in" the music, or inside the music.
In performance, there is an element of phrasing. A soloist or principal dancer can adapt the timing of virtuoso sections in the ballet, to their individual technical highlights or qualities. Some dancers will work this out in rehearsal, and do it exactly the same every time.
And yet, in the less definable area of timing, there is poetry, there is a quality that isn't just good phrasing, that reaches a soulful level. The dancer, the music, the drama, the audience, are all in the same stream.
Such a quality does not require narrative drama to be visible, or tangible. It can occur in abstract choreography as well. It is not just that an experienced performer can "let go". Musicality can be apparent during ballet class when a performer or student is working hard.
Sometimes musicality can be invoked in students as they train. Movement imagery and mental tricks work. For example, giving an allegro exercise in a waltz, and saying "just stay in the air for two/three and land on 1" will bring out a better height and quality of jump. It will not be exactly staying in the air for two beats and landing on the first beat, but it will cause a dance student to reach for that timing and their work gains a quality.
Hearing the "and" between the beats of music is a technical necessity for petit allegro, for battus. But when there are several "ands" in between the beats - where do you choose to put the movement emphasis?
Musicality makes it easier. Of course practice makes it easier, but musicality makes it delicious for those watching, because some can feel what their eyes see and their ears hear, even without drama or emotion.
If you love watching ballet, you know what I mean. If you are extremely musical you may not know what I mean or why I would write about musicality in ballet class. You would ask "doesn't everyone do it like that?"