Some 'compound prepositions' have very different meanings from their primary component, but over at is semantically indistinguishable from at. Over at may always be reduced to bare at. However, as FumbleFingers, the reverse is not true; over at can replace at only if at bears a spatial sense, not in uses like at work or at all events or at a price of.
Over at is a colloquialism, not used in formal written English, and I think its use is mostly a matter of rhythm. Note that in spoken English simple prepositions are usually unstressed, but in compound prepositions the first element usually takes more stress thatn the second, so when a compound preposition is used it that first element always has at least secondary stress:
There's been an ++accident at the ++works!
There's been an ++accident +over at the ++Works!
Sometimes, if it's a supplement, it even takes the same stress as the object:
There's been an ++accident — ++over at the ++works!
If there is a semantic component here, it may be that the compound preposition takes some of the emphasis off of the specific place and onto the more general notion of “not here but somewhere else”. But that's just a guess ...