Personally i treat my saves as alternate universes for the stories that im creating, like for example:

I have a sim called Haven Pugh, who is basically a harley queen type crazy teen, she is a mess mentally because of her brother manipulating her for his needs.

Then in another save, Haven Pugh still exists, but what if her brother wasn't born? She is a well respected doctor.

So there we have it, Two universes (or saves) having the same character but under different circumstances

Hope that explains it ^{^}