In the current SEO age, websites have more expectation of being dynamic with their content. A site that does not have regular updates and additions will be recognised by the Search Engine crawlers as being static and will rank it lower accordingly. The only exception to this rule is possibly a site dedicated for archiving, although in this case you would expect new pages to be added periodically, if not changes to existing pages.
Even if your site has specific products that you sell and you do not see the need to update your site, it is a general rule in SEO that a site needs regular updates to be taken seriously by the Search Engines.
The golden rule is that the internet is for sharing information and making any money should be a by-product of sharing information. If your focus is too heavily on products and sales, and not the sharing of information, there is a good chance you will not get the results you desire with you SEO campaign.
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