macOS Catalina gets rids of the iTunes app in favor of separate Music, Podcasts, and TV apps. It also has the new Sidecar feature which turns the iPad into a second display for the Mac.
Also, with the Find My app you are able to find your devices even when they are offline by using Bluetooth pings to other nearby Apple devices.
Apple is expanding Screen Time to the Mac letting users track their usage across all of their devices for a better overall picture of their time spend using their Apple devices. Photos has also been updated to better highlights your best pictures and Safari include a new startup page with Siri based suggestions.
Just a reminder, 32-bit apps do not work on macOS Catalina as Apple has drop support.