It's a sort of respectful way of gaging your partner's response to a new play. It's a question that allows your partner to say what they feel about something and wiggle out of it if they want to. "Actually, I did try a bit of that once and I really couldn't get into it". It's good for when you have a partner that has a hard time saying no, even when they want to.
It also goes along with role play. Doms talking to their subs as if they are sluts. Acting as if their sub has done it all, and can then be very happy to "be the first" about something.
Of corse the Dom knows that he is the Subs first play partner. He knows the sub has never done any of this. He still going to often ask things this way.

I was just catching up and thr previous chapter he asks mc if he's ever done bobdage before but he's supposed to know mc has never done any of it before. Hes thr only one hes done this with. Snd hes been told this snd been mentioned multiple times. Did author forget. It wasn't even a you haven't done this before right