Can someone explain why people play heal instead of barrier on mid and top?

I cant think of a reason it would be better than barrier 1. There is noone to share the heal with, thus barrier gives you more value 2,Most midlaners have burst damage whitch makes barrier stronger 3. A lot of people play ignite on mid, whitch negates about 40% (sorry if I'm not accurate) of heal's value (also assassin's like kata and fizz have built in griveous wounds) Can someone explain why people still play heal instead of barrier?
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