The 8mm mod plants the rear and and reduces traction rolling.
Now, how you want to go about using this to your advantage is a whole different story. The reason behind the mod is that keeping the angle of the upper link the same by raising both ends equally lowers the roll center, so in that regard it makes sense that people are doing this mod to try and make the rear end more predictable. That is as simple as it gets.

Now for some reasons why you shouldn't do the 8mm mod automatically. The higher the traction, the less need there is (from the standpoint of 'planting' the rear end) to run the 8mm mod. If you are perfectly content with the handling of your car, but are strictly looking to reduce traction rolling, I would look elsewhere (say, lowering ride height a millimeter or two and going to a stiffer sway bar). At the ROAR Nats, Maifield didn't run the 8mm mod because the track had enough traction and he needed the car to transition very quickly from left to right in all the high speed chicanes and flowing corners. There is a point at which lowering the roll center too far creates a lazy feeling car that is slow to transition, so while it would have given him more rear traction, the car wouldn't have transitioned as quickly. If the track was slower with more 180's and fewer switchbacks, I'm willing to bet he would have been running the 8mm mod.