Because sometimes, the generic giant spider just isn’t spidery enough.
Before an apprentice wizard’s master will teach him the web spell, the apprentice must harvest the material components. The master, who has a red hourglass tattoo on his right cheek, sends the apprentice to seek the particularly strong webs of the giant black widow.
Like their small cousins, giant tarantulas sometimes migrate in great swarms. While they are not particularly dangerous at this time, being more concerned with traveling and mating than feeding, it is hard to ignore an onrushing stampede of Gargantuan spiders that reflexively throw barbed hairs at any creature who strays to close.
Giant black widows are typically too large for mites to use as mounts. But they still do their best to coexist with them in a symbiotic fashion. A common trick is for a tribe to misdirect humanoid interlopers toward a nearby giant black widow’s lair to be devoured; they can also command her swarming young after hatching.
—Pathfinder Bestiary 2 256