They are used to create kind of a stair step type of action for pumps. For instance, your pump will be on for 20 minutes, then 60 minutes, then off. 10/20/30 on/off/on would be on 10 minutes, off 20 minutes, on 30 minutes.
It becomes more useful when you want to stagger multiple pumps to have opposing action. For instance:
PM1 10/20/30 on/off/on
PM2 10/20/30 off/on/off
Would give you opposing actions. Hope this helps.