I disagree with "need", i think if previous expansions had scaling at level, the experience would have been better.
I am fine with Wrath zones scaling up to 80 then stopping, Mop 90 etc... But i think scaling has done great things for the time out in the world.
There were plenty of forum posts complaining that there is only 1 max level zone in previous xpacs, which forced them at the time to have pockets of max level mobs in lower level zones like in WoD... that was bad, i think scaling has been one of the best things to come along with Legion.
I am raid geared so most of the zones are trivial now anyway but mobs are not meaningless, can't run through suramar city naked and one shot everything... but i can't die if i don't.