What is "iTunes Device Support Upgrade"?
The iTunes Device Support Update addressed a rare issue that could cause problems with updating and/or restoring devices through iTunes on macOS. This update was not required for Windows versions of iTunes.

This update was recommended for any macOS users who use iTunes to update or restore their iOS devices. It ensures that these updates/restores work properly.

[UPDATE]

Below is a screenshot taken from the Mac App Store app running within El Capitan showing the iTunes Device Support Update and its description: