Cruiser is right, and it can improve the accuracy of the machine because it allows you to run the gibs more tightly.
Chris D, we're not talking about lapping a machine hand scraped to precision. We're talking about machines usually just milled and sometimes that not very well. The amount of lapping involved is not sufficient to remove enough material to reduce the accuracy, but it is sufficient to allow the ways to run more smoothly and hence to let you adjust the gibs tighter, which results in more accuracy.
Such are the compromises on these cheap machines. Yeah, it's horrifying to "professional machine tool builders." But in the end, go with what works.
Cheers,
BW
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