Over time, even the most reclusive of vampire covens will leave clues to their presence. And where there are clues, there are ways to find the truth. In order to better mask themselves, the Cronvangr coven specifically chose to make their lair within a preexisting Frostbite spider den. After all, few would think it strange for travelers to go missing when they pass by the home of gigantic man-eating spiders. With the rise of the Dawnguard, in addition to the presence of the Vigilants of Stendarr, they need to make themselves as discreet as possible
In exchange for leeching off their presence, the vampires ‘herd’ the Frostbites. By keeping them away from ambitious targets such as merchant caravans or large parties of travelers, they reduce the likelihood that the Jarl will issue a bounty or that the Companions will be summoned to cull them. They will also help the spiders hunt during the harsher winters, and sometimes use magelights to play with them.
Fun fact: Brethyn has herded the current broodmother since she was a hatchling. Her name is Ouakam, ‘swimmer’ in Dunmeris. She spent most of her youth causing undue trouble for Brethyn by crawling into his pitchers of drinking water and refusing to leave.
Eventually, he just started keeping a separate bowl for her.