Some explanation is good. Besides, the 4th and 2nd worlds were pretty big, with only a handful of survivors, so there's lots of things that could exist that would be a complete surprise (i.e. the GM can create anything that fits his campaign).
Having the ED background (hey, GMs aren't bound by copyright!) helps flesh out the world, and provides some neat plot hooks.,.. IF you care to use them. But most folks in the 6th World are just trying to get by without worrying about ancient history, and your runners don't have to encounter a single 'weird' thing in order to make for an interesting campaign.