As with almost anything we buy or use, overtime it becomes outdated and there is often a need to upgrade. This is the same when it comes to websites. 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. Over time technology advances and what a consumer is looking for can change so therefore you may need to update your website to reflect this.
Sometimes businesses require online calculators or live chat areas adding, for example. Adding functionality can be tricky on a live site, so always make sure that you have a recent back up of the site before any changes are made. You should also try and implement any changes on a test version of the site prior to setting it live to iron out any creases that may occur during the process.