Avid travelers or just caring people probably noticed that when meeting passengers or strolling through the cabin, flight attendant holding his hands behind his back. However, it still does not check the seat belts and does not ask to switch off phone.
The answer is quite simple: before take-off flight attendant is required to count passengers. Counting the lead or at the entrance or in the cabin, when everything is already seated. In this case, helps here such counter-clicker: