Imagine going out to visit your customer.
It's the first day of the new year. You have your brand new wardrobe at your disposal. You don your brand new shirt and suit. Put on a smashing pair black shoes and head out the door. Now let's say that you never returned home for a year. Sounds ridiculous. But keep with me. Your clothes would look great for a week or two. But would be walking off you after a month. Those brand new shoes would have seen better days. All in all you would look tired and unloved.
And the same goes for marketing messages - especially on a website. Keeping the same message, images and information on a website for longer than a few weeks, could mean that your message becomes lost - or never looked at - or somewhat bedraggled! Keep content fresh, keep your ideas flowing and above all - keep people coming back to your website.