The writing is on the wall: Apple doesn’t want us downloading or managing iOS apps on our Macs anymore. Most of the changes in iTunes 12.7 revolve around features Apple removed, including ringtone management and iTunes U. But the kicker is the removal of the iOS App Store and support for app syncing. This means you won’t be able to click a link on a Web site on your Mac to download an iOS app to iTunes, nor will you be able to use iTunes to choose which apps are on your iOS device. You can’t even customize Home screen layouts from within iTunes. All those tasks must now be done on your iOS device. You can still back up your iOS devices to iTunes—but if you have to restore, your apps will be downloaded from the App Store. Sorry!
Assuming Apple continues its previous practices, once macOS 10.13 High Sierra comes out, it will become impossible to download 10.12 Sierra for the first time. That could be awkward if you want to upgrade an older Mac to Sierra at any point after High Sierra ships, since you won’t be able to get Sierra then. To ensure that you can snag a copy of Sierra in the future, open the App Store app on your Mac, type Sierra in the Search field, and click the Get button for macOS Sierra (it’s about 5 GB in size). It downloads to your Applications folder, and the installer launches automatically. If you don’t want to install Sierra right away, choose Install macOS > Quit Install macOS to quit the installer. What’s important is that Sierra is now registered to your Mac App Store account, and you can get it again from the App Store’s Purchased screen at any time on any of your Macs.