White Plume Mountain: rival parties add spice to the scenerio. First, you have to find the three stolen artifacts and return them to their rightful owners. That's a scavenger hunt. Rivals add an urgency to the party's movements because there is no reward for second place. Second, rival parties can add gravity to the dangers. If another party attempted something and failed, their dead bodies could serve as a reminder of that danger. Thirdly, if there is an NPC that the party is supposed to trust or help, being a member of a rival party explains why they might be there. I think of the Lone Wolf game books. In every book, Lone Wolf ends up by himself to survive the situations. But he often encounters people along the way for good or for bad. Not all adventures warrant a rival group, but many do and add a little extra sauce to the game.
Rival PCs and known NPCs add depth to “You walk in a room and see a body on the floor.” Their first thought can shift from checking for loot to seeing if they are ok.
