The majority of them are for my save. It makes it easier for people who already have a version of my save, and also for console users. They can just download the households, put them in the houses they belong, and continue playing their current save.
I just recently learned that in my save alone I have 554 sims (including pets), and those are just the housed ones. It's probably closer 570 with the Not in World sims. I decided to attempt to rotate between all of my sims, which admittedly I have never done. That's how I discovered the number.