One of the seldom mentioned centerpieces to iOS 9, is a new system wide font called San Francisco, which replaces Helvetica Neue. Apple has mandated that this new font will be unifying every device from the Apple Watch to the MAC. Developers now have access to San Francisco, giving them the ability to use it in all of their apps.
Helvetica, was created in Switzerland in 1957, when there were no digital devices. Helvetica is widely used by many companies as the corporate type even now, and no doubt it will be used in the future as a great classic font.
San Francisco, on the other hand, is a modern font. It will change the typefaces dynamically according to the context. It is a kind of “Digital Native” fonts for the digital age.
Apple is not the only company to develop and release a major new font that was designed for multiple pieces of hardware. Amazon generated a new font that was designed for an e-reading experience called Bookerly a few months ago, while Google crafted a new font that replaces Droid Serif and is very noticeable when you are reading an e-book from Google Play Books.
I think anyone who has an e-reader, digital comic or magazine app on iOS should start to use Apples new font. The key reason is so that your app remains modern and does not break immersion.