Why do writers make the familiar strange on purpose?

Because we stop seeing whatever we are used to. In 1917 the Russian formalist Viktor Shklovsky used the word defamiliarization, or ostranenie, in an essay called "Art as Device". It names the artistic technique of presenting common or ordinary things in an unfamiliar way so that audiences gain new perspectives. For Shklovsky, art makes objects unfamiliar and makes forms difficult, so that perception takes longer. He warned against over-automatization, which would make a person function as though by formula.

One of his examples came from Tolstoy. In the story "Kholstomer" the narrator is a horse, and it is the horse's point of view, rather than a person's, that makes the content of the story seem unfamiliar. By adopting the perspective of a horse, the story exposes some of the irrationalities of human conventions.

You can also defamiliarize the way a story is told. The formalists separated the fabula, the basic story stuff, from the syuzhet, the way that stuff is shaped into a concrete plot. Laurence Sterne's Tristram Shandy is the example Shklovsky cited. Its narrator begins with his own conception and does not reach his birth until the third volume. The plot digresses, jumps around and even puts effects before their causes, which slows down the reader and makes a familiar story strange.

Film plays the same game: Citizen Kane begins with its hero's death and tells his life through flashbacks. A strange order makes you look again.