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.