Indeed. Reading is virtually mandatory for expanding on your worldbuilding. We'd actually say that some fandom setting require almost as much worldbuilding as full on original settings do. They certainly require more specialized research if you want to stick true to that universe's lore.

Mish-mash cultures are fun. Both ones mixed from existing ones (or extinct) and those mixed with fictional ones. Sometimes, historic cultures lack that single element you need to make the fictional culture "yours". That is when the fictional cultures built by other authors can come in handy. Unless your imagination fixes it for ya, of course.

For example... You have a classic culture of mounted warriors. Nothing special about that. What happens when you replace their horses with carnivores? How does that change your culture?
Stereotypes are also fun to play with. Like the russian bear cavalry. You don't want to mess with them.

We could probably go on for hours with examples.