The unnamed interviewer is clearly not that important in the grand scheme of the universe—he has no name, after all. But he is somewhat important to this story. After all, he's someone who has always lived with robots (Introduction.29), which makes him quite a contrast with Susan Calvin, who has seen robots progress from not much (the Talking Robot in "Robbie") to almost everything (the Machines in "The Evitable Conflict"). So, because he comes from such a different generation, the interviewer has to put Calvin's stories in some sort of context for himself—and also for us, at the same time.
In other words, we wouldn't recommend writing a paper on this interviewer as a character—how would you even describe him? But he is a useful tool for Asimov to use to frame these stories in the book since he's so different from Calvin.