Bags are checked in security, and you may think you're good to go once you make it through. However, once you board your flight, flight attendants also zero in on your luggage, specifically airplane carry-on bags. They try to pay close attention to passengers' bags in case they go against any policy or notice someone trying to sneak something on the plane. They want to ensure nothing looks suspicious, of course, for safety measures.
Flight attendants do a lot more than pour drinks and hand out snacks. They have extensive emergency training and must remain attentive and proactive at all times because the safety of the passengers is in their hands.