For adjusting swing weight.  Shaft construction, shaft length, and grip selection will vary the swing weight given a fixed weight of a club head.  If you wanted to go with a Winn-Light grip and keep the swing weight the same, you'd need to put in a lighter weight.  Most of the clubs have a D2-D3 swing weight.  You can also use a heavier bezel if you like to feel the head more.  Some of the UL drivers have shaved more weight off of the grips and shafts and end up with a D7-D9 swing weight.  Which is perplexing as the population most likely to benefit from an UL is also the most likely to not be able to well control a swing weight that high.