The new APFS for Mac is in response to make OS X compatible with I products (phones and tablets). The file system makes efficient use of flash and SSD.
Notice that it's not compatible with spinners at this time because of the nature of how the new file system stores blocks of information.
I wonder if this means that Apple is planning to ditch the spinners (courage!), but not until all the spinners they have in stock are sold. That would mean no updates for iMacs and Mac Mini's until then.