It may be that it took quite a while to get your website all up and running, so once it is you may be tempted to leave it for a while.
If you do not have any new services or products to list you probably think there is no reason to add new content to your site, but if you do not do this you run the risk of losing any rankings that you already have. Google and the other search engine giants like to see that your website is updated regularly. This shows them that you are proactive with the website and the information on there is current. If the web pages are not update or no new ones added then it will quite possible demote the website and push it down the search engine results. Over time there will be changes that come about in your industry and you may wish to add these to your website or at least refer to them to show your visitors that you are aware of any current industry changes or news.
Recovering from a loss in rankings can be tricky and can take a lot of time, mean while you could be missing out on a large amount of potential customers. You want to try and avoid this at all costs.