As with most things, over time they become dated and need to be updated. A website is no different and often with technology moving so fast, a website will need updating on quite a regular basis.
If your website is over 12 months old and you have not really changed much apart from adding a bit of new content to the site then you should seriously consider carry out an update. It may be that you just update the design, changing the way the content is delivered or adding in scrolling images etc. You may decide to add videos to the home page or change the way the navigation displays. You may also want to update the functionality of the site by introducing calculators or little widgets that tell the customer information that is useful to their visit. For a holiday website this may be the current exchange rate or weather predictions for this time of year.