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it's good to learn how to play a couple characters in each role really well. You need to know 2-3 in each role though for various reasons.. If your champion gets banned, what do you do next? If the enemy picks a counter to the champion you always pick before you choose your champion, you need to have another champion you can use in the same role so you don't counter pick yourself. For example, if you always play fiora top, you don't want to pick fiora if the other team has already chosen a malphite top because you would get absolutely smashed. You need to have secondary and even a third option to choose from.
You also don't want to burden your team by only knowing one role. What if someone else only knows how to play the same role you only know how to play, then you guys are in a bit of trouble because one of you are going to have to play somewhere you suck at. It's always better to be that versatile guy that can play well anywhere.