Explaining it to the wrong person

I am currently reading D&D adventure modules in preparation for my campaign after the current adventure. And I noticed something curious: Many of the descriptions appear to be addressed to the DM reading the adventure, but not to the players. For example there are descriptions of what happened in some place long ago which explains why the place is now inhabited by some monster. But what the players will see is just the monster (which they will then presumably slaughter), with no way of find

World of Warkraft...