Two reasons in my mind. No.1, because they are looking for a brand new lifestyle in another city. What they want is finding a place where seems totally different from they used to live in. Experiencing another type of life I can say. Like a coastal city for example, many tourists come there mainly because of the seaside. No.2, they have just heard about the city or just they are wondering why they haven't been to the city before. Like Shanghai or Beijing for example. They just have heard lots of stories of the place but never been there before, just give it a try, no specific reason. That is it.