[csours]

If you have multiple parts in an assembly, each with small deviations from the specification, those deviations add up. Now add some substantial load (like a whole airplane), and you can have a real bad time.

Edit: in this case it looks like it was airplane skin panels, some (most?) of which may be stressed members - meaning that it's not a cosmetic piece, it's a load bearing piece. If you have multiple panels with tiny deviations, that changes the loading of the whole structure, potentially leading to warping, flexing, and premature failure.