The purpose of a question like this, and of the classic FizzBuzz question, is to be a lightweight test of whether a candidate can actually write code or not. That's all. It provides a negative signal if they fall completely to get anywhere close, and nothing more.
There are alternatives, you could ask them to explain a piece of code they wrote on their own before the interview, any piece of code would do. I suppose that could be gamed which is why most companies opt for FizzBuzz.
Personally, once I started thinking about these kinds of questions like this, I had less issue with them. Interviewing people who failed to write FizzBuzz opened my eyes too.
There are alternatives, you could ask them to explain a piece of code they wrote on their own before the interview, any piece of code would do. I suppose that could be gamed which is why most companies opt for FizzBuzz.
Personally, once I started thinking about these kinds of questions like this, I had less issue with them. Interviewing people who failed to write FizzBuzz opened my eyes too.