Use year only matters when/if you have to cancel a trip.
That's because use year determines your banking deadline - 8 months following the start of your use year. If you regularly travel at the same time of the year, it's best to have a use year that begins no more than 6 or 7 months prior to your vacation time. That's because if you have to cancel a trip, you'll want the option to bank the points if you can't use them before they expire.
For example, if your use year is August, you have to use or bank your points before the end of the following March. If you schedule your vacation for June, you would only have about a month to use the points before they expire. Since you are past your banking deadline, you do not have that option.
If you scheduled your vacation for December and had to cancel, you'd have until the end of July to schedule another trip or until the end of March to bank the points into the next use year. That extends the life of those points.
Over the course of your membership, your travel habits may change. What starts out to be a "perfect" use year may turn out to be not so perfect. Also some members travel several times a year ans there isn't a "perfect" use year that covers all of their trips. Other members are fortunate in that they have not had to cancel a vacation and so for them use year doesn't matter.