In a recent comment, Jonathan linked to his collection of articles on Gygaxian Naturalism. Reading ravyn's Ecology for World-Builders from that list, I suddenly understood why the first edition AD&D Dungeon Masters Guide has so many rolls for random encounters in wilderness forest: prey animals.
I always thought that the number of encounter checks listed for forest travel on page 47 of the DMG was excessive—six times per day-night cycle—especially during long overland treks. However, these present an opportunity for the conscientious world-builder. The typically low-interaction encounters that result when normal animals are the "wandering monster" allows the DM to inject colour and setting information without unduly slowing the pace of the game.
A well-designed, naturalistic random encounter table isn't just full of monsters. Non-aggressive creatures—prey animals—filling many of the Common table slots adds verisimilitude to a game world. Having the party stumble upon a Shambling Mound makes a forbiddingly-described forest more concretely dangerous in the players' minds. In the same way, spotting deer bounding in flight away from the party establishes the game world as a living place with minimal effort.
Encounter a prey animal such a deer can immediately establish a number of assumptions for the players: there is foraging here; something eats the deer; we can hunt if our supplies run low; monsters we fight are the dangerous exception; we can seek deer paths to move more quickly; we are not alone out here. Giving some though to the local predator-prey relationships and reflecting this in an area's random encounter tables can make this effect nearly effortless during the game. Including prey animal encounters, whether random or scripted, also helps set the tone of a game in which players are expected to resolve events in ways other than combat.
Obviously these effects are not limited to systems like D&D that include random encounters. These same principles can be applied to narrative and scene-based systems to good effect, but I suspect that using mundane details to invoke associations and convey colour is somewhat more obvious to GMs and players familiar with the modes of play that those systems thrive on.