Shave tires to get rid of tread.

On a treaded R compound, about half the life of the tire is the thread blocks, but once those are worn off, you are essentially left with a slick with 2 rings and about 3/32nds tread life. Once it gets down to this, the tire is much faster. Also, the more tread you have, the more the tread distorts and chunks. If you plan to race in rain, then you would want some extra tread and the RA-1s are a better rain race tire.

Another advantage of shaving it to this point vs just waiting for it to wear down is that it may wear uneven to the point that when you get down to 3/32nds, part of the tire may be worn much more giving you a short life on full slick mode