You're either pressing a capture button on the device, or using a voice command to take a picture or video. So it's designed to send a social signal to others that you're doing it.

When a video is being recorded, the image appears in the screen. If you're at face-to-face speaking range or a little further away, it's actually not that difficult to see in the image in the display. And even further off you can see what looks like the screen being lit up.