what the advantages to tuning the spring weight? Heavier = more pressure? Stronger engagement?

Basically the more power you make the more likely the clutch is going to slip. Stiffer springs will make it less likely to slip. The trade off is the stiffer the spring, the harder it is to pull in the clutch lever. Chris having 73 pound springs on his drag bike, he doesn't use the clutch that much. On a trail/dune bike I think the 73 pounders would wear your hand out in a short time of riding.