Your Use Year determines the time period within which each allotment of points can be used and also your banking deadlines. Many of us prefer having a UY that is right before our primary travel times. That way, if we ever have to cancel a trip just a month or so in advance, we would still have most of our UY left to use the points for something else. We would also be able to bank the points returned from the cancelled reservation.
For example, if you always go in August, then an August UY would be perfect for you. If you may want to go in June or July some years, then a June UY would be better. The worst UY would be one that is right after your primary travel times. For example, if you had a Sep UY and had to cancel an August trip 31 days or more in advance, you would be past all your banking deadlines and have only until Aug 31 to use the the points for something else before they expired.
Use Year does not affect when you can make a reservation. You can book at your home resort 11 months from your checkout date regardless of your Use Year (and 7 months out at non-home resorts).
The reason I mention cancelling 31 days or more prior to check in is that if you cancel 30 days or less from check in, the points go into "holding" and cannot be banked, and can only be used to make reservations 60 days in advance. So if you cancel a stay within days of check in, it doesn't matter what UY you have, you will not be able to bank the points.