んですが and differences in nuance with んです

Question

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:

XXが苦手なんですが

Example sentence from this question:

私は歌を歌うのが苦手なんですが、どうやったらうまくなれますか???

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?

Answer 1

The explanation you first read is wrong.

You use のだ forms when what you are saying is background of the preceding context, beside pseudo-imperative usage.

For examples, if you just ask if someone gives you something, you say くれますか、くれませんか？ or so. On the other hand, when you see someone holding something and reaching out his/her hand to you and you want to confirm if it's a gift for you, you say くれるのですか？.

In your example, it would be also natural even if it was plain …苦手ですが… but the speaker is feeling that the fact that she is bad at singing is reason or background why she's asking there.