MMC matters when you want assembly protection with bonus tolerance. Example: a clearance hole is Ø10.00 to Ø10.10 mm, and you specify position Ø0.20 at MMC. The hole MMC size is 10.00 mm. If the produced hole measures 10.06 mm, the bonus tolerance is 0.06 mm, so the total allowed position becomes 0.20 + 0.06 = 0.26 mm. That extra allowance helps the pin still assemble even when location drifts, because the hole gets larger in the direction that improves fit.