CHtuulikki this is why I like the Formative Moments reward traits that arrived with Adventure Awaits. I add these to all of my new Sims to give them some history and background. Traits like "Learned from the Past" or "Childhood Grief Survivor" are good ones for a Sim that has just been created but that has a back story where they've suffered a loss for example. Then there's the outdoor activities traits for my explorer Sims like "Action Packed Childhood" , "Childhood Collector" or "Childhood Naturalist". There's the family and social traits like "Super Sibling" , "Thoughtful Child" and "Was a Best Friend". It does take a while wading through all 50 of them but I'm finding it's helping me create Sims with a proper background whereas I don't use the Story function in CAS because it just creates quite random results. These Formative Moments traits help me create a more rounded Sim and give me ideas for who they could be and what kind of past they have. So as an example I have 2 Sims I've created since that feature was added that are sisters bonded by childhood grief and who were inseparable growing up. I've played them before but now I can give them the Formative Moments traits to match their backstory.
Sometime I also create family members and dynamics and split them off into other households or even kill household members off or divorce them at the beginning. It would be interesting to be able to just add a solo divorced Sim from the start though. Or add Education history (without playing it) because at the moment they automatically say high school graduate in the social panel.