Helper springs have a very low rate and collapse under the car's weight. Their purpose is to keep the main spring tensioned when the wheel goes to full droop under hard cornering (lifted wheel), over bumps, and on the jack.
Helper springs are required in any coilover setup in which the mainspring looses tension at full droop - that is most coilover setups. If the following formulae is true, you need helper springs:
Corner weight / Wheel rate < rebound travel
Without helpers, the main spring may not re-seat properly and will cause a sudden change to that corner weight/height.
Correct spring divider hardware is required between the main and helper springs. Helper springs are available in a variety of diameters and lengths.