From a reader -
I used to monitor the HAM 2-meter frequency which is for local radio communications. Most of the people asked location by saying, "Where are you at?" I always wanted to ask, "What's the difference between 'Where are you?' and 'Where are you at?'"
The word where, by semantic nature, includes the preposition at, so asking someone where they are is the same as asking "at what location are you?"
That said, it would be redundant to ask the question, "At what location are you at?"