An interval "cycle" = rest time + effort time e.g 20 seconds / 40 seconds = 1 cycle
Interval timers ask you to input two values, one for rest and one for effort. I like to think of the two aspects being important for training an exercise. Each exercise needs the doing of it and the recovering from doing it parts.
I like the workout to end with the beep beep beep beep at the completion of the last exercise. But that only happens if there are no more intervals left on the timer. So I start with a rest period, which also give me set-up time.
So basically, there's no other reason except my OCD 😛