Actually, vapor production is important to some for the same reason throat hit is important to me: It's what they need to convince their brains that it's just as good as, or better than, analogs, and it's what keeps them from going back. For others, it can be the form factor of the device -- it has to look and feel roughly like a cigarette for it to work for them. Still others value specific flavours, like tobacco blends. Different people most value different aspects of smoking and vaping, but it is the presence of that highest priority aspect that keeps them going. Vapor may seem like a purely cosmetic feature of vaping with little practical value, but for some, it's critical that this visual stimulus be present and copious.
Whenever I'm talking to someone about what device and juice is right for them, I always ask, "what aspect of smoking do you think most attracts you to it?" It's not a question smokers have ever had to ask themselves, but it's an important one to consider, because it's one of the key points that will determine what devices and juices will satisfy.