The mainspring affects hammer tension, which affects ignition.
The rebound spring affects trigger tension, which affect the revolver's trigger return and safety function.
In other words: Too light a mainspring and the gun may mis-fire.
Too light a rebound spring and the trigger may fail to reset, AND the safety system will fail to operate.
In the "J" frame revolvers BOTH springs are coil springs.
You get the most benefit by using SLIGHTLY lighter springs for both.
After replacement you MUST test-fire enough ammo to be 100% SURE of reliable operation.
My standard is 100 rounds of standard factory ammo, preferably the SAME ammo you'll be using for "business".
Dropping a new spring set in, popping off 2 or 3 cylinders full of some cheap practice ammo and calling it good to go, isn't going to do it.