Duty rigs ensure that you get paid a minimum amount per duty period. The industry standard is 1:2, meaning you get at least one hour of pay for every 2 hours duty. Thus at least 7 hrs. pay for a 14hr. duty day, 4.5 hrs. pay for a 9 hour day, etc. This encourages efficient scheduling by management, otherwise they can have you sit on your arse at the airport almost all day and not pay you. (Cleveland, anyone?)