Apple explains the need for APFS by writing:
HFS+ and its predecessor HFS are more than 30 years old. These file systems were developed in an era of floppy disks and spinning hard drives, where file sizes were calculated in kilobytes or megabytes. Today, solid-state drives store millions of files, accounting for gigabytes or terabytes of data. There is now also a greater importance placed on keeping sensitive information secure and safe from prying eyes. A new file system is needed to meet the current needs of Apple products, and support new technologies for decades to come.
Instead of slapping on encryption after the fact, APFS natively supports it right from the start. In fact, two features that are currently present in Apple devices are incorporated into APFS: Full Disk Encryption (which was introduced with OS X 10.7 Lion) and File Data Protection (which was borrowed from iOS). APFS supports AES-XTS and AES-CBC and can be configured for “no encryption, single-key encryption, or multi-key encryption with per-file keys for file data and a separate key for sensitive metadata.”
While APFS can obviously be used on traditional hard drives, it has been specifically optimized to take advanced of newer solid state drives which are all the rage these days. APFS uses copy-on-write design and I/O coalescing to both improve data reliability and improve performance for SSDs (Macs) and flash storage (iPhones, iPads, Apple Watch).
Other features include Space Sharing (allows different file systems to share free space available on a physical volume), near instantaneous cloning of files and directories, Fast Directory Resizing, and read-only Snapshots (for efficient backups).
While APFS is being introduced in the macOS Sierra Developer Preview, there are some limitations to APFS in its current form. It can’t be used as a startup disk, file names are case sensitive, Fusion Drives (hybrid HDD+SSD) aren’t supported and the file system doesn’t support Time Machine.
With that being said, the Apple File System is expected to be released next year and will be featured on nearly every computing device that Apple makes ranging from the Apple Watch to the iPhone to the Apple TV to the Mac.