Halliday & Matthiessen (2014: 175):
yes and no may function as part of a textual Theme (like oh, well). Here they are continuatives and serve to signal that a new move is beginning, often but not necessarily a new speaker’s turn; they have no speech function of their own, and therefore merely reflect the current polarity – they are not selecting for positive/negative (and so cannot bring about a switch). In this case they are almost always phonologically weak.