To program iPhone apps. AFAIK, Until the recent Xcode beta you couldn't push code wirelessly to your iPhone, and even now the feature in the Xcode beta is pretty slow. Also I enjoy charging my iPhone with my macs. For a company that takes pride in seamless connectivity within their ecosystem I find it surprising that you cannot buy an iPhone + MacBook Pro and connect them without adapters.
You absolutely can; just not with the cables they ship with. There are lightning to usb-c cables available from both Apple and others now.