The next time you go to see live jazz in a club, and the band is playing original compositions, look closely in front of the musicians. Sometimes there'll be stands holding sheet music. There's nothing wrong with this per se, especially if the music is a bit complicated. But sometimes there'll be no need for stands, as the musicians have memorized the material. It's impressive, but it also signals a certain commitment, one borne of having rehearsed and performed together often. You frequently see this in tight bands that know what they're doing.
Charlie "Bird" Parker was one of the most influential musicians of the 20th century. In his brief life, Parker created a new sound on the alto saxophone and spearheaded a revolution in harmony and improvisation that pushed popular music from the swing era to bebop and modern jazz.