Group Level price books are designed to allow users to set a whole group of items found within a classification or attribute to a set discount code. The same apply for where the price book is set. All items found within a classification and attribute will be set to the discount for as long as the price book is active, versus setting it for individual items.
Merchants can also use price books to move their slow moving inventory. Let's say, it's summer time and you still have many jackets and scarves left. You can set a price book assigned to these specific categories (or sub categories) that would be sold at a lower price compared to your other products. Merchants that have seasonal products that are out of season can be discounted automatically for different stores and location. So for the time that the discount is active, merchant can simply ring up the items and have them automatically discounted.