A website is often an online profile of your business. It may be purely there to show information and images of the products and services you offer, or you may have an ecommerce website that actually allows you to purchase through it. All websites should be updated on a regular basis for customer experience and for security reasons.
A customer wants to be able to read the most up to date information, so for example, if you have a product that has recently gone on sale then you should reflect this on your website as soon as possible. Also if there has been additional options added to the product or service, add these to allow your customers to add them to their basket.
Security is another reason why a website should be updated. It is not only the content that should be added to but also the code may need to be tweaked or the content management system updated to allow if to function as it should and to be less likely to become victim to security breach.