Quote from The Swift Programming Language, which answers your question:
“Swift’s compiler performs four helpful safety-checks to make sure that two-phase initialization is completed without error:”
Safety check 1 “A designated initializer must ensure that all of the “properties introduced by its class are initialized before it delegates up to a superclass initializer.”
Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itunes.apple.com/us/book/swift-programming-language/id881256329?mt=11