Flex layouts give you a power never before seen in PrestaShop. They will let you specify a different template for different product or category pages. Now instead of having sale banners for products that are on sale, you can have a totally different page arrangement or layout for those product pages. With category pages you can use different layouts per category as well.
This can create endless design possibilities for your site to help drive sales and look different from your competitors. At the same time you can also use these templates to A/B test product page or category page layouts. Then you will be able to see which templates convert more sales to your shop.