The rebound slide has to: 1. push the trigger forward, 2. Rebound the hammer (retract it backwards after firing) against the tension of the mainspring, and 3. reset the cylinder stop. The S&W stock spring insures that all of this will get done, no matter what the circumstance and operating environment are. Fooling with that spring when you don't know what you are doing can get you into deep trouble.
The Wilson spring is coiled from different wire, and not ment to be trimmed. If you do you will end up with a spongy feeling trigger return, and that isn't good. The kit provides you with several springs so that you can find the best one for your revolver, and keep in mind that for each pound you reduce the pull you also lose some reliability. Take your pick.