I have read that when you use んです, you're implying that the listener already knows what you're talking about and you're just kind of emphasizing the conclusion. However, I have also seen some examples where んですが is used for a situation that the listener is probably not aware of, most notably:
Example sentence from this question:
The asker is posting in a QA forum where no one is aware that they're bad at singing. They then go on to explain their exact circumstances after they had used なんですが. As I type this I realize that this is probably a retarded question, but does んですが allow one to follow up with an explanation without any implied foreknowledge, while plain んです requires said foreknowledge?