I like to think that, at least in the cases of premades who are in households in town and not just homeless sims created to fill roles like pizza delivery, fireman, etc., it was to make them a bit more interesting or give a potential player ideas on how to play them. For instance, maybe a sim has a boring business job but their lifetime wish is to be a five star chef. If they're unplayed, they're kind of like that person who ends up doing something all their life just to pay the bills, but it was never what they dreamed of doing. Then, when you play them either by picking their household or marrying/moving them in, you get an idea of what you can do with them, and maybe the disparity sparks a creative idea for a story for you to play out.