I would assume that the reason for the requirement to enable ADB for the USB connection is that the used app transmits the video data via IP.

By default Android devices don't support IP traffic via USB. Only via ADB it is possible to establish an IP communication between Android phone and PC through USB (using port forwarding - a feature provided by ADB).

Apps can make use of the USB without ABD however the development is way more difficult and the chance that it does not work on certain devices is high. Comparing it with a connection via ADB the latter is easier to establish, works reliable and does not require extra development work on Android side. Hence it is the simplest available solution.