Read these forums. Some people really like the larger nicer screen of the iPad Air, and some people (like myself) really like the compact form factor of the mini. It's great Apple produces both models to satisfy both groups. As for myself, I didn't even see the point of a tablet until I saw the mini, perfectly sized for slipping into a suit jacket pocket, yet large enough to (at least for me) comfortably read pdfs.
P.S. Whoops, I misunderstood your post. I thought you meant it didn't make sense for Apple to have both, that is, to offer both models. You meant it didn't make sense for a consumer to have both. (I don't know why I goofed; your title was clear). I only have and need the mini, but I won't speak for others.