Wandering monsters end up serving several purposes, providing different things at different levels. Some games cling to them, others have dropped them entirely, which seems to suggest that there’s something valuable here with room for improvement. What’s going on?

Don't roll a 28 if you're in Gévaudan.
Narrative Carrot
From the storytelling perspective, the wandering monster can be a great hook on which to hang any number of things: hints about the current location, clues to other areas, recurring enemies, plot coupons, you name it. The randomness can inspire new storylines (“What are hobgoblins doing this far south?”), personal quests (“Them’s scaly, and Oktar needs a new pair of shoes!”), and local legends (“If you see a troll by the bridge, don’t mess with it.”). Sometimes the inexplicable manticore is just a nice change of pace to break the routine.
Aside from whatever stories may or may not arise from a particular wandering monster, the simple presence of mobile creatures helps create the impression of an organic environment. The forest without wolves is too empty, the fortress without guard patrols unbelievable, and while a good computer simulation can plot out the wolf pack’s hunting grounds and the complete patrol schedule, the wandering monster table does the trick for the tabletop. From the player’s point of view, there (ideally) shouldn’t be a difference.
All of this is Good Stuff for the players.

Don't roll a 29 either.
Mechanical Stick
The problem with all of that “Good Stuff” is, of course, how it generally takes the shape of something that’s trying to eat your face. The wandering monster is a high risk, low reward situation: an encounter that the party may or may not be ready for, probably starting at a tactical disadvantage, with little to gain and everything to lose. From the player’s perspective, it can be seen as simply a waste of time. This is why a check on the wandering monster table is one of the penalties for poor judgement – making too much of a ruckus in the castle, dallying too long in the hallways, sleeping in the woods instead of in a house like a civilized person, and so on. It’s something to be avoided.
In other words, it is chock full of Bad Stuff.
Moving Your Donkey
It seems the whole point of the wandering monster is to move the game along and keep things interesting, increasing environmental depth (the Good Stuff at the player level) while adding danger (the Bad Stuff at the character level). How could that idea be expanded? There’s probably a way to incorporate friction, or something much like it, in the mix somewhere.
Functionally, the wandering monster table is the environment’s reactions to the party’s actions. The party gets six turns to move, then the dungeon gets a move. If the party slips up, the dungeon gets an extra move. The base assumption is that the dungeon is hostile to the party, which is perfectly reasonable.
That leads to two questions:
- What if the dungeon has other move options besides sending monsters at the party?
- What if the dungeon isn’t hostile?
Those seem like good topics for the next couple of posts.