Internal software update servers allow you to reduce the amount of bandwidth used when distributing software updates from Apple. Instead of each computer downloading updates from Apple’s Software Update server, updates are only downloaded from Apple once per server.
Using an internal software update server also allows you to control and approve updates before you make them available.
Disclaimer
The Software Update service has been replaced by managed software updates, which can work in conjunction with the content caching service in macOS 10.13 or later. Jamf recommends updating operating systems on computers and mobile devices using managed software updates. For more information, see Managed Software Updates. For information on the content caching service in macOS, see Intro to content caching in Apple Platform Deployment.