Maybe you're setting up a drawing stream and want your audience to see both your face and your artwork at the same time. You spend time getting the lighting just right. One camera is pointed at you, the other is positioned perfectly over your sketchbook. Everything looks ready to go. Then OBS decides it only wants to recognize one camera. I've run into this problem more times than I'd like to admit. In fact, most of the time OBS has no problem detecting a single webcam.