Over time websites can become stale and outdated and it is important to regularly update not only the content but also the design and functionality. You may have had a website designed a few years ago and at the time it did exactly what you and your customers needed it to but over time, as technology advances and customers become more aware of exactly what they want to buy, you need to ensure that you update your own website to reflect this.
It may be that you need to add a login area for your visitors to view past orders, or a tracking facility so they can see when an item should arrive. All of this can improve your conversions on your website and increase sales.
Adding functionality can be tricky on a live site, so it is advisable to create a copy of the website and work on that first. This will allow you to test it thoroughly before you swap it over to the live site. Even if you have tested your work, always ensure you have taken a back up of the current site in case anything goes wrong and you need to revert back to it.