Essentially, the redesign appears to serve the same purpose as the current layout, displaying personalized recommendations based on your purchasing habits, browsing history, offers, and other signals based on your shopping activity. Today's change simply builds on that formula and includes larger, brighter graphics, more dynamic product curation and groupings, and a greater emphasis on horizontal scrolling for the various collections and sub-hubs you'll find. (It also frees up more space for larger, thinly-veiled displays.)
For example, sports fans might see a “Thursday Night Football” ad next to a refill of their favorite pre-workout, while parents might see toys, children's books and perhaps the new colorful Kindle and be tempted by a diaper sale.
Amazon says it has been iteratively testing various parts of these UI changes for some time. The company is announcing a new “Buy Again” hub that makes it easy to stock up on frequently purchased goods, for example, and I've been watching this change (but not the others) for some time.
These recommendations have also been available before and worked similarly in various forms, but I personally find the current experience seems too random and scattered to be useful.
Amazon hopes that the upcoming changes will remedy this and ultimately offer you more products that suit your tastes and needs – and maybe squeeze a little more money out of your wallet along the way.