For more Sidecar details, see Here’s how Sidecar will extend your Mac’s display with iPad and which apps are compatible | 9to5Mac
"But Apple’s blurring of device lines wasn’t limited to iPads becoming more computer-like. There were also several introductions that highlighted how the iPad can become a more useful computer accessory. Most notable of these was the debut of the new Sidecar feature in MacOS Catalina that will let you use an iPad as a secondary monitor for your Mac. While there are certainly cheaper options for dedicated monitors, the ability to let you use your iPad as a secondary display on an occasional (or even regular) basis is something that many Mac users will undoubtedly find very useful. In an age of increased multitasking, there’s never enough screen real estate, so options to extend your desktop and apps across multiple screens make a great deal of sense.Apple Blurs Lines Across Devices | Tech.pinions
Interestingly, because Sidecar also supports Apple Pencil on the connected iPad, it’s almost like bringing some level of touch-screen support to the Mac. To be clear, it only works with Mac apps that currently support stylus input (think graphics apps), but it can add a Touch Bar, even to Macs that currently don’t have them, and will likely lead to other touch-enabled features."