Besides serving, or sometimes selling, beverages, meals and snacks, they need to take well care of every passenger. Flight attendances have other duties including demonstrating how to use the safety and emergency equipment and provide assistance in case of an emergency. It is really an exhausting job requiring a lot of hard work.
Sitting is considered as a violation of the rule as well. It is never acceptable to show their tiredness to passengers either since that will badly affect their travelling mood. Hearing all these, do you wonder how they can be in the best state all the time on the flight?