Your use year (month) is simply the month that your annual allotment of points is renewed. It also determines your banking windows.
USe year can be important if you have to cancel a vacation that is scheduled to occur late in your use year.
For example, let's say that you have an August use year and schedule a vacation for the 3rd week of June. If you cancel that June vacation more than 31 days from arrival, the points are returned to your account. However, the points expire at the end of the use year - in the example, that would be July 31. If you cannot reschedule your vacation before then, the points would be lost.
Now let's suppose that you scheduled a December vacation. If you cancelled that vacation more than 31 days in advance, the points would be returned to your account. If you couldn't use the points before they expired (again, the following July31), you could bank them and have another whole year to use them before they expire.
Most members prefer to have a use year no more than 4 or 5 months prior to their usual travel times. However, vacation habit change over the course of the years, so what might be a good use year now, may not be so great later on. Keep in mind that it only matters if you have to cancel a vacation that is scheduled for late in your use year.
HTH. If you need more words, post again.
Best wishes -