I never had a ONS as I have some scars and have some confidence issues about them. So for me, it's necessary that a guy knows that the scars exist and the story behind them, so that I don't have to be scared that someone will interrupt the sex and ask me where the scars come from (one on my neck and one on my upper leg basically right next to my vagina).

This might be a special case, but I think the answer is still relevant for your question: Sex is an intimate experience, so for having enough trust, it helps to get to know each other, share stories you don't usually share with others and to build up trust. I'm not saying that a relationship is necessary for that, in my eyes, also a FWB situation is much better when you try to get to know each other some more.