I start new saves all the time, on whim, often because I don't want to play this or that version of something else I was doing with a townie household, im the save with a whole new household or character to focus on.
Like if I have played the Vatore siblings, Caleb and Lilith, but I want them to just be their unaltered default townie selves not under my control, in the world I start s new household with. I don't always want a townie household to ne already played by me, altered by me, I want them to be unplanned NPCs for my new Sim to interact with, so that whatever happens with them, is untouched by me, and not up to me.
Otherwise, if a save gets too played, it feels like nothing can be a surprise, like I'm playing all the parts, so of course nothing happens unless I make it, which is pretty dull.
So most of the time, if I want to play a whole new idea, I want a fresh, untouched world, unaltered by my hand, to play my one controlled household against.
Not sure if that's clear or confusing.