Making semiotic sense of an annoying stylistic tic: So says more than you might want it to.
This is apparently is an example of semantic bleaching, similar to the process that turned very and really (and more recently literally) into intensifiers. The OED lists so as an “adv. and conj.” glossed as “In the way or manner described, indicated, or suggested; in that style or fashion”, with examples going back to the 9th century. Over the centuries, if the bleaching theory is correct, a sense emerged that’s something more like “in relation to the issue described, suggested, or presupposed”.
“So” also seems to indicate a connection between the interviewer and interviewed, a suggestion that the answer really is going to address the question. But often, the interviewed answers a different question, snd the particle becomes a rhetorical backhander. Compare it to using “Well …” in the same context. “So” indicates that the response is canned, being delivered by rote – which also appears in the general tone of voice and cadence. “Well” can suggest the response is more thoughtful and tailored for the context.