It's used for both. Take for instance Super Mario Bros 3, different banks holds different data for the various enemies, and the same banks also contains the subroutines of logic for those enemies.

That's why, even if you hacked so that Goomba's death behavior should point to the same as that of a Drybones, it would fail as the subroutine for that behavior isn't in the same bank as the Goomba.