Things a random encounter can do (+ table-making advice)

Purpose of an encounter

In OSR systems, the classic random encounter turns time into a resource: every turn spent in the dungeon (or every day in the wilderness) means a chance of a confrontation, which would drain party resources and potentially kill PCs.

But an encounter can accomplish various other things... (In fact, this is vital in systems like D&D 5E where encounters are no longer resource drains by default and must justify their own existence, lest they become yet another annoying old artifact to be tossed away.)

Some of these include:

1. Present a potential challenge or opportunity (the most common one)
2. Creating cohesion in your setting by acknowledging, tying together, or foreshadowing disparate aspects of your world to make it more cohesive.
• For example, if you've slotted someone's crystal-man dungeon into your hex map, then the encounters in the region around it could involve crystal-men, since they're a thing now.
3. Introduce plot hooks and underline ongoing event
• For example, soldiers coming back from The War That's Going On Right Now carrying whatever they've pillaged.
4. Include some local color
• For example, merchants and performers on the road, heading to the nearby town's Summer Festival.
5. Emphasize setting themes or make its lore accessible
• For example, in a world with an underlying theme of corruption and sin, this could mean flagellants, pious men and women on a pilgrimage seeking redemption, or holy knights looking to cleanse the land. An opportunity to explain the state of the world or its history.
6. Present tough decisions: a one-time chance for a trade, a dangerous but rewarding choice, or (parsimoniously, for the right kind of groups) a moral quandary.
• For example: traveling plague doctors will pay very well to have a strong test subject drink their latest experimental plaguewarding potion, but there kiiiind of is a small chance it'd kill you. Will you volunteer? And will you let them test it on that chained-up man, who claims he was sold to the doctors by his family?
7. Establish a new concept or mechanic
• For example: in a world where runestones exist and can be bought, the PCs may encounter a runemancer willing to explain and demonstrate how rune magic works. Alternatively, this can be as simple as saying "hey guys, this faction/this type of monster exists". A good opportunity to gauge the players' reactions, in any case.

etc. Other reasons can include "I've got a specific mental image in mind that I want to bring to life" or "I just freakin' love dragons, man."

It's good to know what you're trying to do with an encounter result, and to remember the various way that powerful tool can be used.

Quality and quantity

You don't need three different dinosaur types or four kinds of evil humanoids unless you've got a good reason (for example, four important factions at war in a location, or four evil humanoids with a degree of importance in your setting). In my experience, an ordinary encounter table can get away with approximately six results, doubly so if they are supplemented by subtables for variety (e.g. "what the monster wants", "what's the monster doing", etc).

So, focus on one result and try to make it interesting and re-usable: the same barbarians can serve a dozen times if they're back with loot on the first encounter, having an internal feud on the second, running from a monster on the third, etc. In fact, running into the same barbarians multiple times builds consistency, establishing them in your players' minds, and allowing your players to apply their increasing knowledge of the monster/NPC with every new encounter.

For one-time encounters/events (like "Glorb the wizard is about to get killed by his apprentice") you should probably have one or two replacements ready for after it's used up (although the idea of a wizard regularly hiring apprentices that inevitably try to kill him on the road sounds hilarious, if you can keep it fresh...)

Odds and variety

In my opinion, it's better not to reuse that same old "d100 swamp encounters" table you grabbed off your favorite system for every swamp in your game. What makes this swamp special? Give it its own encounter table and tailor the results for it, even if it means going down to just d10 results.

Weighted tables are cool in theory, but it's important not to put mundane stuff at the forefront and hide your really interesting results behind a low-chance (<5%) roll. Your players aren't going to make fifty treks through the same area unless it's a megadungeon, they might not even make five, so design for good sessions instead of designing for satisfyingly pseudo-realistic numbers. In fact, a campaign starring three separate encounters with a dragon may grow a lot more interesting for it.*

* Unless these dragons can't always be avoided or escaped, in which case, uh, don't do that.