If a vehicle that has been created as part of the ambient population is suddenly required to take on a more important role, such as the car being involved in a fire fight. It is important to generate new entities that don’t seem overly generic or unlikely given what the player knows about the game so far. Generating data can be thought of as providing a memento to read from just in time. JIT mementos are faked mementos that provide a false sense of continuity by allowing pseudo random generators or hash functions the opportunity to replace non-zero memory usage mementos when the data is unlikely to change, or be needed for more than a short while.
JIT mementos can also be the basis of a highly textured environment with memento style sheets or style guides that can direct a feel bias for any mementos generated in those virtual spaces. Imagine a city style guide that specifies rules for occupants of cars, that businessmen might share, but are less likely to, families have children in the back seats with mum or dad driving, young adults tend to drive around in pairs. These style guides help add believability to any generated data. Add in local changes such as having types of car linked to their drivers, convertibles driven by well dressed types or kids, low riders driven almost exclusively by ghetto residents, imports driven by young adults. In a space game, dirty hairy pilots of cargo ships, well turned out officers commanding yachts, rough and ready mercenaries in everything from a single seater to a dreadnought.
JIT mementos are a good way to keep variety up, and style guides bias that so it comes without the impression that everyone is different so everyone is the same. When these biases are played out without being striclty adhered to, you can build a more textured environment. If your environment is heavily populated with a completely different people all the time, there is nothing to hold onto, not patterns to recognise. When there are no patterns, the mind tends to see noise, or mark it as samey. Even the most varied virtual worlds look bland when there is too much content all in the same place.