A recent question asked about the impropriety of "Where's it at?" The question started me thinking about when at is allowed with where. My first thought was that ne'er the two should meet: at is always extraneous.
Some verbs, though, seem to give more room for the pairing. "Look at where" or "point at where" are far less strident than "sit at where" or "is at where". Still, "look where" and "point where" are far cleaner.
It seems to me that at is more permissible when the subject is not at the location stipulated by where. In the two examples above, the subject is one place and directs her attention "at where". The at becomes a way of keeping distance between the subject and the place.
What is the function of at when it precedes where?