Juxtapositions are kind of like the foils of language. Put two things next to each other, and their similarities and differences are much easier to spot. It's like when you park your '87 Toyota Camry next to that shiny new BMW in the parking lot.
Sometimes juxtaposition is used just for the sake of creating absurd situations. In The Importance of Being Earnest, much of the comedy actually comes from that bizarre juxtaposition of characters and events.
Speaking of bizarre, the Gothic genre just loves to juxtapose the sacred and the profane. When Dostoevsky puts crosses on the bloody body of a murdered pawnbroker in Crime and Punishment, we get the point. Nothing is wholly sacred.
Even (or especially) words can be juxtaposed—you'll find this a bunch in poetry. Joseph Conrad, who really loved to wax poetic, even threw us some juxtaposition in the title of one of his books, Lord Jim.