With the arrival of version 12.7, iTunes is no longer the place to browse the App Store, nor to download and manage iOS apps. If you’re using it and were in the habit of downloading iOS apps to your Mac, you may now have a large and redundant archive of files.
These downloads are found in ~/Music/iTunes/iTunes Media/Mobile Applications, with apps varying in size from a few megabytes to several gigabytes. Back up to external storage and you can trash the folder, because these apps will never again be updated on your Mac.
(You can, however, later put one back on a device, by dragging it from Finder to the device in iTunes.)
If you can’t bear to part with the App Store or any other features removed in iTunes 12.7, download and install iTunes 12.6.3 instead.