Feel free to take whatever non meta pick you want
but you still have a role, I dont care what champion you play as long as you fill that role as best you can
would much rather have a illaoi supp or something like that, that builds relics and sightstone than have a sona that starts dorans and rushes lich bane
it's not just supports, though that is the most clearly defined role
another example would be if you are playing malphite top, and you have a taric supp and rammus jung, then you know what? You go right ahead and build yourself some ap if you think that will help you win lane and make up for our comps meh dmg.
But if you are malphite top and i see you building roa into deathcap when our team has no tank, i might get a little salty