The surprising reason people say it.
With the decline of “whence” and “whither,” it fell to “where” had to pick up the slack. It had to be equipped with a “from” to do the job of “whence,” and sometimes with “to” to do the job of “whither.” At this point, locational “where” is the odd one out. We have “where to,” “where from,” and just plain “where.” It was almost inevitable that an “at” would creep in there at some point to level things out.
“Whence” and “whither” have been in more-or-less steady decline since at least the mid-1700s, and according to the Merriam-Webster Dictionary of English Usage, it was only later, in 1859, that “where at” was first noted in a Dictionary of Americanisms. They agree that “where at” is almost never used in formal writing, and attribute its growth in popularity in the 1960s to the idiom “where it’s at” that I began this episode with.
I’ll end with one situation in which “Where it’s at” is actually perfectly standard English. Don’t believe me? OK, get ready: Someone asks you, “Where was the crow sitting?” Your answer: “On the branch is where it sat!”
This article was written by Neal Whitman, who blogs at literalminded.wordpress.com. Check out his blog.