The early 2000s X-Men film series expanded superhero' films complexity and character development. The Superman films of the 1980s and Batman films of the 1990s, while entertaining, tended to reset characters so that the next film could start over from the same place. The X-Men movies proved that audiences would welcome a more dynamic interpretation of the material, with storylines that continued from one movie to the next. Jean Grey's emergence as the Phoenix was one of these arcs, set up in X2: X-Men United in order to be picked up in X-Men: The Last Stand.
Source：https://www.cbr.com/x-men-last-stand-jean-grey-death-impact/
