Why do I need to play squishy champions jungle top and support

I kind of don't get why people are telling me I need some squishy chamions for these lanes, why? For one thing the mid and adc already do great damage, and typically the mid is all you need to delete carries, so i still don't get it. I have only ever heard I need a assassain or two for other lanes just in case the mid is someone like heim, and I need some carries for other lanes because.... they get pentas sometimes?... Oh and another thing, how do I pick meta champions without feeling bad. (I swear I avoid zed like the plague) Also if this is the wrong board, I apologize I didn't know where you went for questions.