The Halo games have a tradition of giving players who complete the campaign on Legendary a special extra custscene at the very end of the credits.
In the case of Halo 4, the Legendary cutscene happens to contain the only real explaination of the Didact and his motives in the entire game. If you found the Didact to be poorly handled in the game, go to youtube and check out the Legendary ending. It really helps add some clarity... I personally don't understand why 343 didn't include this info earlier in the game. It would have helped things make more sense :-/