'Such-and-such' is used to refer vaguely to something that doesn't need to be specified.
'So-and-so' is used in the same way to refer vaguely to a person who doesn't need to be specified.
These expressions mean roughly the same as __________ (blank), and are used either when describing someone or something that isn't important enough to mention, or when you simply can't remember the specific thing or person you are talking about.
examples: I must have one of those familiar faces. People are always coming up to me on the street and saying, "Haven't I seen you before? Weren't you with so-and-so?" or "Didn't I see you at such-and-such a place?" And I have to say, "No. That wasn't me."
We were specifically told to meet them at such-and-such a place, at such-and-such a time, but then they didn't even show up.
When conducting job interviews, be sure to ask the candidates how their experience working at such-and-such a place prepared them for this position at our company.
"So-and-so" can also be used euphemistically for name-calling. example: That Jeremy is a nosy little so-and-so.