1: Certain items can only have certain stats, for instance, boots are the only thing that can have speed as a primary stat, so for *most* builds for *most* champs, you want boots with speed

Gloves are the only item that can have crit rate or crit dam (actually amulet accessories can have crit dam too) as the primary stat, so you *probably* want those for your hardest hitters, but it isnt as important for champ who doesn't do a lot of damage in the first place.

2: It depends on the champs role, mostly. Lifesteal gains back health based on the amount of damage they do, so a champ who doesn;t hit hard won't get as much health back. Immortal & regen gear restore health based on thier max hp, so a squishy champ wouldn't benefit as much for those sort of sets.

An HP or DEF champ's damage isn't based on their attack stats, so offense gear/attack stat boosting doesn't help them

Perception is a very good set because it gives speed and accuracy, which are useful to *almost* every champ, and also it an be forged, and with glyphs, making it easier to get a good piece with stats you want than randomly grinding a dungeon until you get the right drop