Because contacting your developer everytime is just a pain!
The complaint that we regularly hear from people who already have websites is that they are unable to make changes on their own. In order to make even the simplest modification, they have to contact their web designer and wait for them to update the website. They also usually have to pay to go through this hassle. Having a content management system is a great option for a website that needs frequent updates.
Most of the time we use WordPress as a CMS (Content Management System) because it's the fastest, most user friendly option but we aren't opposed to building something custom if necessary.