costume rom for the note 4, is it really necessary? topic

before i bought the note 4, i used to own S3.
while the base original rom for the S3 was pretty bad in most ways & custom roms were a must, it's not the case in the N4! i find it fast, no bugs or lags etc.
so i was wondering, when talking about the note 4, why bother with rooting and costume roms, losing the warranty etc?
what's the different from custom roms to Samsung's original rom?