What it comes down to, is that the spring (set up correctly) will help to correct inconsistancies within our set up. In other words, a bad release, possibly inconsistant hand pressure, a nick on the shaft, a feather touching the launcher... see what I'm getting at?
To set it up correctly, you want just enough spring tension so that the launcher will support the shaft (not sagging) when an arrow is nocked and sitting in the braced position. This is the best place to start, but during tuning, you may find you need to tweek the spring tension some.

I hope this helps.