Achemists 'R' Us: The local Alchemists' Guild is low on supplies. Specifically, they are out of acid, the key ingredient to some of their most profitable products (people in this area seem oddly predisposed to lobbing acid-grenades at anything that moves). Far to the west is a swamp, no doubt rife with horde pygmies (
Monsters 36) and
Muck Elementals, which is known to be a breeding ground for acid spiders. Sure, you could just go there and hack them to bits for their acid glands, but how much more fun (and/or profitable) would it be to capture some (by traps or by magic) to slowly "milk" acid from their fangs ensuring potentially limitless (albeit dangerous and ravenous) stream of income. . .
This could easily expanded: acid is only one ingredient the alchemists need. Paut from Electric Jellies (
Monsters 24), the lens of an Eye of Death(
Monsters 26), Bubear spleens (
Monsters 17), "cold organs" from Frost Snakes (
Monsters 28) or Ice Wyrms (
Monsters 38), not to mention various herbs and minerals from nasty places near and far.