I thought it helped the gain a bit by being centered in the dish rather than using one of the offset positions. People have reported getting a few extra points on their signal using it as compared to using one of the slots out of the standard dual LNB mount. I've never tested it myself.
If the standard D500 with dual LNBs is such that only around 90%-95% of the LNB's circle of coverage hits the dish, then centering it would enable it to hit 100% of its coverage. Dish could have designed the D500 to be as small as they could make it and still work sufficiently. So it might not be perfectly aligned for maximum signal from both LNBs.