If you still have no idea why switching to SWIFT is a step that any developer should take, below are three reasons for “Why”.
Another safety feature is that by default Swift objects can never be nil. In fact, the Swift compiler will stop you from trying to make or use a nil object with a compile-time error. This makes writing code much cleaner and safer, and prevents a huge category of runtime crashes in your apps. However, there are cases where nil is valid and appropriate. For these situations Swift has an innovative feature known as optional. An optional may contain nil, but Swift syntax forces you to safely deal with it using the “?” syntax to indicate to the compiler you understand the behavior and will handle it safely.
#2 Fast and Powerful
Swift is a successor to both the C and Objective-C languages. It includes low-level primitives such as types, flow control, and operators. It also provides object-oriented features such as classes, protocols, and generics giving Cocoa and Cocoa Touch developers the performance and power they demand.
#3 Interactive Playgrounds
And new in Xcode 7, playgrounds can contain comments that use rich text with bold, italic, and bullet lists in addition to embedded images and links. You can even embed resources and supporting Swift source code in the playground to make the experience incredibly powerful and engaging, while the visible code remains simple.
-Source: Apple SWIFT: Why and why not? (Part 1)