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Different ranges of gears used different carriers for the ring gear. (I don't know what they are in a 10 bolt) Some of them are thicker than others. A spacer would allow you to run a "thin" carrier with a set of gears designed for a "thick" carrier.
Not to try to butt into your buisness but what are you regearing to? If you not going lower than 4.11 then there are alot of people that will sell you complete 10 bolts for cheaper that the gears new.