Is not because we want to prepare for a recital.
Is not because we have to prepare for an exam.
Is not because we have to meet a deadline.
Is not because we want to impress others.
For an artist, the act of practicing is self-validating. By practicing we want to give back to God (or whatever higher power you believe in) all we got from him. For us who have a calling or a dream to advance our craft, not to practice would be in fact selfish.
The real reason we practice is that we don't have a choice.