Ask HN: Mobile carrier asked me to look at the camera and blink 3 times

I went to the carrier shop this morning to buy an extra SIM card. After providing my ID and the usual information, the attendant asked me to look at his phone's camera for a second and then "blink three times". I felt really shocked, especially thinking about my privacy (I haven't signed anything up to that point). I asked the attendant about the photo, he said "I don't know, it's our procedure."

Does anyone know what that photo is for? What about the blinking? Why 3 times?