IN MY OPINION: AI is for trying champs you've never played before, same with builds. Ranked is for people who want to be able to increase their rank within the ranked community. Normals is somewhere in the middle, for me. Not serious enough that you should worry about losing, serious enough that you shouldn't try random things you've never done before on a whim.
I know I play normals because I want the challenge of other people without the raging and stress of ranked. I want people to do their best and appreciate the work people put into playing, but not flip out when people aren't at their A game or doing everything perfectly.