Most of the time you would want to use flour that has had some time to mature. In some cases, maturing agents, such as potassium bromate, azodicarbonamide or ascorbic acid, are used to speed up the process. In this case, what the miller is doing is oxidizing the dough so that you have stronger gluten. This process is speeded up also if the flour is stored in a warm area. The down side is that the gluten can become too strong. For example, if you use flour that was milled 3 or 4 months ago, and you want to make croissants or any layered product, the dought might shorten back up when you are trying to stretch it out. Actually, this can happen with almost any product, which can be a problem if you expect a certain width of product and end up with a smaller diameter or length and a puffier product (imagine cookies).