First, let me preface this by saying that there is always something to say. I would have a hard time believing that there isn't something you could change or add to your website at any given time.
But...if you really truly believe you have nothing to say, here is why you should keep it fresh:
Your website visitors.
When it comes down to it, that is the best reason you should keep it fresh and up-to-date. The people that are actually coming to look at your site, read your content, and look at your photos.
From a content perspective, you want to be sure that the information that you are providing your visitors is accurate. There are many websites I see where years go by before any content is changed. This has to stop. If your visitors are getting inaccurate information or stale information, they will stop coming back.
From a general update perspective, there is always a small bit of news or education, whether company or industrial, that you can share. Share this news update right with an interesting perspective, you'll keep those visitors coming back and you will educate them. An educated buyer will likely buy more and will be easier to sell to.
From a look and feel perspective, visitors will start to ignore your site when they come back if this doesn't change a bit. I am not saying that you should change your design every week, but changing a photo on your home page would be enough. You have a short time to catch a visitors attention and it can't be done easily if they think that the site they are looking at is exactly the same as it was 1 year ago, 2 years ago, 3 years ago...you get the point.
The bottom line is that your website shouldn't be there just to be there. It should be there for the visitors and provide them great information. Sometimes you have to take an extra step to grab their attention, but it's worth it. Time to start looking into that content management system! ;)