minion dematerializer, wave freezing?

hello i'm not sure i get what minions dematerializer rune is used for sometimes i take it, it help me farm cannon minion so i'm not susceptible to poke when i try to last hit him but when i see pros streamers i often see that they use it in regular minions they do it to freeze/defreeze wave effectively? bonus question, to freeze wave you take out casters first or melee? which of them deal the more damage?
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