As I know, the word "there" has been used as an adverb a lot.
But it's also a noun. Not only can you say "What are you doing there?" but "What are you doing in there?" also works. But what about "at there"?
I was checking some homework from a few kids in my work. There was this question: What did he get at the supermarket?
Nearly all the kids wrote "He got some apples at there." I've never heard people say that, but I'm not quite sure if it's grammatically correct but people just don't say it, or it's just ungrammatical.