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Comic strip of a ghost and a person with the American flag pasted on the head. The ghost repeats "Boo!" in the first three panels without getting any reaction, but when it in the fourth panel says "kg, cm, km, °C" the American gets scared and screams "AHHHH!!!".
Edit: fixed alt text
Least important it may be. But it is the most significant. This scheme follows the conventional scheme we follow while writing numbers - the most significant digit to the left and significance reducing as we move right.
The advantage of YYYY-MM-DD becomes when you add time to it in ISO-8601 or RFC 3339 format: YYYY-MM-DDThh:mm:ss. All the digits are uniformly decreasing in significance from left to right.
This becomes even more apparent if you are trying to sort by time - say, a stack of files, or datetime in a computer. Try doing this with any other scheme.