As a GM, wandering monsters and other random encounters can be difficult to utilize without being a burden on the game. The best illustration of this point I’ve so far seen is in Rich Burlew’s excellent comic, Order of the Stick.  But is it true that the wandering monster is nothing but a boring waste of time? Where did they come from in the first place, why did they seemingly disappear from modern games, and are there valid uses for them?
In the early days of RPGs when DnD was the only game available, wandering monsters served the mechanical purpose of whittling down the PCs’ resources based on the time they took, eliminating the possibility of players working around challenges via time consuming and convoluted plans or camping to regain spells after every encounter . Thematically, they displayed to players that an area was alive and dangerous.
Sadly, the wandering monster has fallen out of favor in modern games. While they can still be found with some searching, they’re no longer as prominent as they once were, mostly due to their reputation as an unnecessary time sink and the hobby’s attempt to be inclusive of as many play styles as possible. However, the noble and misunderstood wandering monster has much to offer your game if used effectively and with some care.
Wandering monsters still serve the function of giving an area a sense of life and danger, but a well thought out selection of wandering monsters can also serve to lend flavor to your campaign. Give careful thought to selecting wandering monsters appropriate to the area in which they are found. Also consider other factors that you may wish to showcase, such as season, weather, or holidays or other special events.
Dispensing clues and adventure hooks via wandering monsters is also a way to greatly increase their usefulness. Even if a particular encounter couldn’t plausibly shed light on the current story arc, dropping clues to possible side quests, hidden locations, or even the overarching story is a great way to get extra mileage out of the humble wandering monster without the hint seeming too forced.
Don’t forget that Wandering monsters don’t always have to be monsters. Weather patterns, tracks, hidden caches, small locales not linked to the story, and NPC allies and personalities are all good alternatives to monsters. These other options are usually faster than a combat encounter and can add the same kind of flavor to your game. These are especially good options to mix with standard wandering monsters. Fighting a monster in a storm, by a roadside shrine, or with a notable NPC are all good ways to make a seemingly mundane encounter memorable.
Wandering monsters also give your players new ways to exercise their roleplaying muscles, and try new approaches without the same risks as story based encounters. In games I have run or played, random monster encounters have led to magic item trades, formation of new trade routes, trophy hunting, unusual modes of transportation, cockamamie get rich quick schemes, and unlikely allies.
While wandering monsters can be an element of your game that requires careful planning or quick improvisation, they have the potential to be much more than time consuming speed bumps on the road to more relevant encounters.