Updating your site is important?

People say that updating your site often is one of the most important things you should do to keep your site at a good rank in google, but i was thinking what if you site is just a sales page of a product and its not really a product that you can keep adding information about ?

You can always add something new, even once in a few months is good. Think out of the box - maybe improve your product description, add some new testimonials etc...whatever.