a question for every role

i know every role has its uses n all but some of my questions lay unanswered here we go, starting with supports, ive always had a question thats never been really given a flat answer, cant supports have potentially the biggest impact in the game?. mages, in a fair amount of ways mages do more damage or basically the same damage as a adc, if so why not just get a second adc, whats the true purpose of a mage?.adc so if a mage does more or the same damage as a adc then despite "carry" whats their purpose then.tanks n fighters, the two classes steal eachothers attributes so why not just make them the same flipping class reffering the attributes eachother has.and jungle my fav but always leaves me pondering, a jungler feels almost no different from anyone else, anyone can gank, anyone can roam(just not that smart with most champs) and why attack the jungle when you can be in lane getting kills?,well i dont have any questions for marksmen and assasin, those are just obvious