It seems to me that any valid number can also be expressed as a String, so I don't know why this function returns a String? instead of a String.
My best guess would be because of the legacy support. This is from the official documentation:
The behavior of an NSNumberFormatter object can conform either to the range of behaviors existing prior to OS X v10.4 or to the range of behavior since that release. NSNumberFormatter Class Reference
来源：https://stackoverflow.com/questions/28103113/does-nsnumberformatter-stringfromnumber-ever-return-nil