GMs have to strike a weird balance in ongoing campaigns. Don’t do too much preparation; so that when players go off the blazed trail you aren’t starting the nights campfire with your notes. But, also have enough structure so they aren’t just spinning on the log flume again and again going nowhere. All while having a little bit of fun while doing it. Many people look to random tables to solve the preparation issue to avoid burning out from creative fatigue. The problem with that is many of the encounters I’ve seen using encounter tables have been horrible; worse than just throwing a dart at the index. There is little life to the encounter and the GM doesn’t understand how the monsters are to be utilized in the scene.
Doing this in a way that won’t bore your players into snoring fits takes some mental preparation. Using random encounter tables is good for a tiny part of creating an encounter: finding a thematically appropriate monster. The table is a good list for this because the creators of the list have taken the dungeons theme into account, but you as a GM have to consider theme in your execution.
So you’ve rolled on the random encounter table and you now have something for your players to interact with. You need to ask yourself, why is the monster there? How has the monster showing up changed things? Do I want my players to try to kill the monster? Do I want the monster to try to ambush my players?
With these questions you start to get the beginning of the scene in place and can begin thinking about how the scene should be developing for the short term. While you can’t dictate how the players will react to a situation, you are setting the stage for a certain style of encounter. The description is truly the set dressing for the scene, a counter for the shop keeper to be behind isn’t a guarantee if you haven’t brought it up and the players have no idea if the shop keeper has a specialty unless they’re given that information.
You now have a monster to put in place, what place is that going to be?
When a room has no description it’s a featureless blue box. Since the GM is in charge of setting the stage upon which the players play, and the players won’t do a single thing with regards to taking cover or doing any sort of tactics if they don’t hear about it, the same sort of thing goes for the monsters. If there isn’t a theme, description, and mode of action for the monsters, then you might as well have the players fighting the Börgies — even droids have more personality than some GM monsters.
Now you have the walls painted in your blue room with the random blue boxes for the players to hide behind, and the monsters will completely ignore them. I’ve said before that a monster needs to deal with their environment, which means you need to actually take into account their environs. How a monster moves in a bare cave is vastly different than in a cellar with barrels and supplies sitting near it or in the middle of a crossroads that the players are approaching. A large bugbear waking to the players presence is going to be very different to the small goblin group outfitted with arrows. While one will smash things out of the way to get close and tenderize the players, the other will stay at range and slowly help the players with their hedgehog costume.
The aspect to this that is most overlook is that just because you’ve gone to Donjon.bin.sh and had it spit out a 36 room dungeon of appropriate level for your players to go through, it will still take several sessions to get through. If the players are tuning out by room 6 it was a waste of everyone’s time. The one question that the GM needs to answer each and every time they sit at the table is “So what?” and the answer can’t be repetitive. If you have trouble answering that, you could be playing Descent or Imperial Assault and have more fun building and painting an AT-ST. Just because the monsters want the characters demise isn’t always enough tension for the players to care.
Randomness is supposed to bring something new to the table, but if the bugbears fight with the same tactics that the orcs and the hobgoblins fight with what was the change? Are the spiders blue instead of red? No one will notice. A single change means nothing unless you make it mean something. Meaning is having it use the surroundings to add to its effects and act in a way that would make sense for that new monster. Creating something different from the last encounter the characters were in front of should be the goal of using randomness. Not just random for the sake of random.
Using random tables can help give the players something that isn’t your usual standby encounter. They can be great, but you have to work at them. Every single random encounter table I’ve seen has one or two options I’d never think of and once I start to ponder how this antagonist would act in a scene it gives me a reason to want to try it.