IME when we reply 'Yeah, no...', the first part isn't an affirmation, it's an acknowledgement that you have heard and understood the question.
Think of it as an informal but intimate way of speaking. If you just answered "No." it could come off a little bit cold or professional. By taking the time to use a few extra words you are showing more warmth to the asker.
Re:The Linguistic Evolution of ‘Like’
by marcus_holmes on Monday November 28, 2016 @07:11PM
(#13059569)
this feels right to me.
"yes I have heard you, but I disagree with your point"
Something like: "did you try it this way?" "yeah, no that didn't work"
To me the initial "So..." is something that was very noticable in conference talks etc a few years ago. (Probably replaced now with "Whassup?", even when talking to a non-existant audince, i.e. by YouTubers)
They also covered what is, to me, an even weirder linguistic phenomenon, the common and growing use of "yeah, no" at the beginning of a response to somebody: http://www.slate.com/articles/podcasts/lexicon_valley/2013/0...
I use this "yeah, no" construction myself quite a lot. I have a strong instinctive understanding of how to use it, and what it means. But I don't think I could consciously provide a decent explanation of why I'm saying both yes and no about something.