The way a random encounter in the dungeon goes in D&D (certainly in my games, at least) is usually something like this:
- Party is doing stuff
- DM rolls d6, 6 for random encounter
- DM generates wandering monsters
- Surprise roll
- Reaction roll
- Initiative roll
- Return to exploration loop
The other thing missing here is that a rule has been ignored - the random encounter distance rule, another roll between generation and surprise. Nominally should be 2d6 * 10 feet in the dungeon, in ACKS at least. That's a long way (in expectation). That's outside torch radius on average (30' of bright light, 20 more feet of shadowy illumination). It's also outside of average monster infravision radius (60'). If you, like me, tend to not have 70' hallways, generally that's going to be around multiple corners or through multiple doors. This is not typically a "the monsters come around the corner or through the doorway, roll for surprise" situation.
The reason this rule is ignored is straightforward - tracking runtime-generated state outside of PC line of sight is a hassle. If you're using a graphpaper map, there isn't a good way to track groups of monsters moving through the dungeon. Digital tools could probably handle this better.
But if you're willing to pay the price to track these 'blips' outside of PC detection range, the atmospheric and gameplay benefits are, I suspect, significant. They turn random encounters from "fire and forget" into lingering threat, things just outside your vision, eyes reflecting your torchlight. Waiting in the dark for you to make a mistake, stalking you, looking for an opportunity to pounce (or maybe just to eat your dead). A heavily-armored party might not even be able to close with and engage such groups due to speed differences. I suspect these lingering random encounters might encourage the use of thieves in the shadowed zone (where they can quickly pin down or drive off such enemies; pickets) and other "lightweight" play. Multiple random encounters might lead to multiple groups of uncommitted creatures - morlocks ahead, morlocks behind, nowhere to run. Depending on the monsters and the ecology of the dungeon, multiple "open" random encounters might fight among themselves (presenting an opportunity to players) or join forces. Players might detect them with listening or detect evil... blips on the paladin-scanner.
These things in the dark become a source of tension, a source of potential energy for the dungeon. The other shoe, ready to drop.
On reflection, this difference in use is reflected in the old naming - "wandering monster" instead of "random encounter".