Because I continue to work on my software.
While working on my software, I have to keep my software up to date to the most recent version of macOS. The latest releases of my software tends to focus on OS X El Capitan , macOS Sierra, macOS High Sierra and soon macOS Mojave.
The macOS Sierra 10.12.1 update improves the stability, compatibility, and security of your Mac, and is recommended for all users.
iOS 10.0.3 fixes bugs including an issue where some users could temporarily lose cellular connectivity.
iOS 9.3.5 provides an important security update for your iPhone or iPad and is recommended for all users.
For information on the security content of Apple software updates, please visit this website : https://support.apple.com/kb/HT201222
The future would be macOS during the next WWDC ?
Mac OS 9 > Mac OS X > OS X > macOS ?
The OS X El Capitan v10.11.4 Update improves the stability, compatibility, and security of your Mac, and is recommended for all users.