It's a fashion statement, conspicuous consumption (owner has money to throw around), a political leaning/association, and an assertion of the owner's masculinity. To be clear, this behavior gets all my hate, not only because all of these things are encouraged by our economy and advertising, but are also directly supporting a lot of toxic traits and behavior.
My brother-in-law has one of these trucks. He still lives with his parents, doesn't tow or haul anything, and works as a janitor. He's paying $350/month on the loan just so he can feel cool.