Let"s take the example of coinage. Here the user assumes that a coin has a minted edge that is 90° to the surface and is assumed to be very sharp. But this is not true, because the coin has a very slight belly, and this is measured.
Worse still, the minting may displace some metal which may protrude, perhaps "right at the bottom" of the coin. In this case, the outermost tip of the displaced metal would be measured and not the actual edge seen "from above". Therefore, it is very important for the understanding of the results to know where exactly a measurement is taken. In the case described, it might be that this protruding chip is a flaw that is to be detected and measured.