This cover protects the USB connectors and Ethernet port on the Raspberry Pi while keeping room for nano USB adapters.
It can also be used on the side with the HDMI connector when it's used headless for example. It snaps into place securely and can only be removed when the case it opened.
Note: You can use these covers in combination with the Cyntech case for the RPi4, but the ethernet port isn't accessible anymore.