The Maniac Difficulty was made for those who Likes a Challenge.
If could start with very Strong Personas, that will make any Battle a breeze, well, that won't be considered as a challenge.
Starting from the beginning is the hardest way to go, can also lead to a fresh start!
The Battles will be more tense, if the Party is weak, and can be defeated easily!
The difficulty will not directly affect the story though, it still can cause Stamina issues, if will be going to Tartarus too often...