Simple version, the better dampers control the movement of your suspension more efficiently, getting rid of the energy generated by the motion of the wheels without allowing it to upset the chassis. This will keep your chassis more settled during hard driving, which means you can go faster ;) . If you have ever driven a car with worn out suspension (or an older GM car with their pillow suspension) and had it pogo across bumps, then you have experienced what a suspension acts like when it doesn't dampen out the hits. If you are going with shorter/stiffer springs you will need the better dampers as well, the stock ones aren't designed for the lower stance and added energy the stiffer springs store, and they won't work very well. If you get a kit like the Tokico HP set, the dampers and springs are (supposedly) designed to work together. My experience with mine says they are.

If I am off base on this then someone please let me know, sometimes my explanations aren't the best.

If you are interested, here at the item numbers for the two Tokico HP kits on eBay:

4580786685 $300 plus shipping, approx 2 days left
8005226436 $400 plus shipping (buy it now, Matty Mooling from this forum),
approx 2 days 20 hours left

Good luck!

Trust chiefmg - spaketh correctly.