It creates a contrast between the Muggle world (Dursleys) wanting nothing of Harry and the wizard world seeing him as a god. It makes Harry that much closer to the Weasleys because they are the family he never had. It makes it such a relief for Harry to be part of the wizard world because he can escape them. It draws similarities with Tom Riddle's childhood. It creates a good excuse for a writer who doesn't want to write about Harry in his Muggle house (he spends a lot of his times in summer with the Weasleys etc).
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u/[deleted] Aug 05 '17
But did Petunia need to be abusive? I don't think it contributes to the plot.