Helper springs are designed to take up slack, and are mainly use with single perch suspension such as KW.
In a single perch design: KW, Ohlins, etc.
Your car sit on the springs, and you adjust the springs platform to adjust the ride height.
So at the lowest setting of the main springs (when there is 0 pre-load), if that isn't low enough then to go lower will mean the springs will be loose.
In order to take up this gap, helper springs are design to fill the gap.
As with all single perch design, the lower you go, the less compression travel you have, and the more likely you will hit your bump stop.
Dual perch design: MeisterR ZetaCRD, Ohlins DFV, etc.
As the suspension are design to sit at 0mm pre-load, and the ride height are adjusted via the lower bracket (by changing the length of the damper), helper springs are not needed.
Some dual perch coilovers I see with helper springs which I feel odd, as it mean you are taking compression travel and throwing it out the window.
The ONLY time a dual perch coilovers need helper springs is when your springs are so stiff that you do not have enough droop travel, then you bring a helper springs in to give you more droop.
That is typical of race suspension as they are lighten and have very high springs rate.
So you need the helper springs to give you the droop travel you need.
That is why some coilovers have helper springs, and some don't.
But just throwing helper springs in (as I see in some coilovers brand) do not make a suspension better... if anything it often makes it perform worst.
P.S. forgot to add... rally car is a different kettle of fish as they have extremely long damper stroke travel and need the helper springs to take up some of those travel.
But that is outside the scope of usage for most users on here.
Jerrick