The point of getting a separate device is similar to using a VPN - to use proprietary and invasive software without getting the personal data stored on the main devices sent to third parties. The business device still can send some data (like WiFi connection info, approximate location and microphone recordings) but the personal data should be safe. It may not be a viable option for very high threat models though since Windows can have backdoors that the government might use to inject malware into all the devices connected to the same network.