It is a way of judging age and versions of a lens. The Canon 50mm f1.8 Mark I is easy to tell because of it's metal mount vs the plastic mount of the Mark II.
But can you tell me the differences between the versions of the Canon 400 f2.8? No? Me neither, but the date code lets me determine the version and from there I can determine the specs of that lens. This can help in determining a fair market value, and whether it is a lens that I would want.