When iOS update time rolls round, you probably run through all the devices in your home, downloading those multi-gigabyte updates over and over. If you have a few iPhones, plus a couple of iPads, it all adds to a lot of data, and a lot of waiting. New in macOS 10.13 High Sierra is Content Caching, which stores these downloads on a Mac, so they only have to be downloaded once.
This doesn’t only save on internet data usage. It also makes it way faster to update several devices. And it’s not just updates either. Content caching can also cache iCloud documents, your photo library, and app updates.