Like, I'm fond of spiders, so I think it's cool that the Brotherhood just happen to have a chill pet frostbite spider in their Sanctuary, but I'm not really sure what the point of having her around is. Lore-wise, it's obvious that they feed people to her in order to dispose of them, but from a gameplay standpoint she seems kind of superfluous.
I'm probably just being dumb, but I kind of wish there was more to her. Like, if you could milk her for venom periodically, or recruit her as a follower and sic her on enemies, that'd be cool. As it is, I don't see what purpose she has, other than being decoration.