I have seen this many times over the years - the sales process glossing over or ignoring this. I did not buy on my first sales pitch and went to do research for a couple of years... glad I did because when I bought I insisted on the use year matching my vacation planning cycle.
Here why it's important: You need to have the time to plan and make reservations and travel arrangement before your banking window closes.
Our year starts August 1. We prefer to travel in Late August through the fall. For years before out daughter was school age we traveled Sept/Oct/Nov and now we go last half of August most of the time. We make reservations for our stays just after the use year begins. If our plans must change, I have 7 more months to decide what to do... and then I can still bank my points.
Now imagine my use year were February 1. If my August plans don't work out, I must bank by end of September! Little flexibility for other plans. Worse if I had plans for November, and have to change - I could be past my banking deadline!