It's the creation of a market, just as the myriad listings of varieties for a given date creates the demand over and above simply ticking the date box. Sure FDC should mean FDC, but even with two nominally similar grade coins, everyone will have a preference for one over the other based on their perception of what is before them. Personally, I think the concept of giving or chasing micro-grading is irrelevant as it often takes preference over the contents and is entirely subjective on the opinions of the grader.