The reason why characters who can see stats that aren't relevant to their classes, is so that players don't get confused about seeing stats on items that aren't anywhere in their character information.
If a player who makes a warrior and sees intellect on a ring that drops, he won't have a single clue what this stat means. He'll look in his character information window, and it won't be written anywhere.
For the "have a separate section for useless stats", it's to save space and make it more neat. Instead of
Useful stats :
Useless stats :
it's just grouped up into one section. When you hover your cursor over INT, it now says "Provides no benefit for your class".