I agree with most of the people here: Backstory is as relevant as your players choose to make it.

In my case, I like having backstory so that when the players ask questions like, "Where are all these cultists coming from?" I can answer with in-game information like, "they've been recruiting for 20 years," instead of, "well, because I needed this many cultists for a level-appropriate encounter."

Granted, 7 times out of 10, the players will miss or ignore your exposition. But, for the 30% of the time where they don't ignore it, it's better to have it than not.