Use year only comes into play when you're banking points. You may bank 100% of your points within 6 months of your use year beginning. At from 7 to 9 months you can bank up to 50% and in the 10th month, you can bank up to 25% of your points.
If you travel at the same time every year, a specific use year can save you points if you need to cancel. For instance, we go the last week of May/first week of June. Our use year is February so if we had to cancel, we could still bank 100% of our points into the next year.
Hope I didn't confuse you more. Use year is not all that important in the grand scheme of things...