I had someone ask me recently why I was reading a book about marriage customs in the Middle Ages. They knew I don't write historical novels and wasn't taking a class, so why spend time on the book?
The simple (yet completely incomprehensible) reason is Avatar.
Avatar, Firefly, Star Trek, Robin Hobb's Six Duchies and Bingtown series, Pern, Dune ... I could keep going, but I expect I've made my point.
They are all set in imaginary worlds. But all of those worlds have a basis in ours. Avatar played with colonialism. Firefly with slavery and civil war. Star Trek with just about every known culture and cultural movement and social challenge.
I like playing in imaginary worlds when I write. I enjoy a well-crafted universe.
It's more work than you might think to create an entirely fictional world. It's a lot of work to keep things consistent and it drives me nuts when it doesn't happen. (My greatest irritation: Nickelodeon's Bubble Guppies. Granted, it's a kids' show. But that doesn't excuse having your characters stymied by a boulder in their path. Newsflash to writers: they can all swim around it.)
My favourite type of fictional worlds are ones which look almost exactly like ours, but with a slight twist. Buffy the Vampire Slayer or Grimm are great examples. In order to make both the altered world and the secret cultures/organizations within it comprehensible, a writer needs to be able to understand how our current cultures and organizations came to be.
There is nothing inevitable about any kind of culture or any aspect within it. Whims of fashion can calcify into tradition. Practical decisions become embedded as taboos. Misunderstandings can be perpetuated for generations.
To pick a completely mundane example: trousers. Pants were not worn in the ancient world. Men and women both wore robes and skirts. Pants came from the Germanic tribes, who lived in colder climates with pointier bushes and trees. If Rome had not been sacked by the Germanic Vandals, pants would likely not have become a standard for clothing. Just one little detail, but it changed a great deal about how history evolved.
That's the kind of stuff which fascinates me. It's accidental, almost coincidental, yet has a profound impact.