Content caching is a great idea, especially if you have an always-on (or often-on) Mac in your home or office. Instead of having to download the same stuff over and over, you only do it once. Then, any other devices on your network pull their data from your Mac, instead of from Apple's servers. That's probably a boon for Apple's data bills, but for us it could make a huge difference to update speeds. When this feature is switched on, both Mac and iOS content is saved locally.
How to set up content caching in macOS High Sierra?
• Open Sharing from System Preferences.
• Unlock the pane. Click the padlock icon in the lower-left and authenticate.
• Configure the Content Caching service.
Shared content: store apps and software updates.
iCloud content: store iCloud data, such as photos and documents.
Share Internet connection: share this computer's Internet connection and cached content with iOS devices connected using USB.
• Switch it on. Click the checkbox on the left of the service name to enable it.
Keep in mind that content caching will download all content requested by any client device, so the service will consume vast quantities of storage space unless you limit the storage. You can inspect how much storage space is being used on your Mac by the caching service by clicking the Options button. You can also use Cocktail (Preferences > Caches > System > Content caches) to flush/clear content caches.