I frequently play legacies, which means that over time (with some mod help) not only are all the original sims long gone, but the file has gotten bloated with several generations worth of sims. So I find it helpful to "start fresh" to clear out the accumulated crust.
Also sometimes I want to play the stories that require interacting with the premades (who, as I said, are usually long gone by the time I finish a story). Especially if I want to replay Strangerville (weird, I know, but I like it. It's like watching a favorite movie combined with a retelling of a favorite folktale) - those sims don't stick around forever, and while the do get replaced, it's nice to see it "as intended" sometimes.
Other times, I may need a particular world set up in a particular way to tell a particular story, that conflicts with how my current save is set up.(like my test save for HSY, where I've de-aged all the base game Adults and young Adults... only to realize that I now have way more than 18 teens in my save, and that's before the HSY sims get added.)
That and honestly, it's just cleaner. New story, New save. Like a blank document.