Setting goals, visualizing them and putting them in front of you on a daily basis is a tried and true tool (among many others) of the most successful people/organizations throughout history. In sports and in all walks of life, the most successful people tend to visualize their achievments and goals, and believe them to exist long before they ever physically obtain them. So... putting up banners to remind a team of it's past, while simultaneously setting an image of the ultimate goal (the Lombardi trophy) slightly higher than those banners, is a smart and all encompassing message to the players. This alone will not make the team win, but it would be foolish to brush this move off as meaningless.