If you have a ‘news’ section, for example, what do you think a visitor to your site will think about you and your business if the last piece of news is six months old? Or if you refer to key members of staff who have now left? Also, the search engine robots regularly visit sites (especially if your SEO is done well) and new content is what they are looking for.
Top Tip: Put a slot in your diary every week – it only needs to be for 5-10 minutes – and look at your site as if you are a potential customer. Anything need changing?