Why build your website in WordPress?
First, because there’s no reason to be dependent on your web designer every time you need to make a small change to your site. Not only does it cost you money, but it’s not very exciting for the web designer, either. You need a content management system, which is a web publishing tool that separates your content (words, pictures, video, etc) from the site’s design and function, so that you can add new pages or pictures, for instance, without worrying about messing up the design, or change the design without messing up the content.
There are other open-source content management systems, and they have their strengths. But since 2007, when everyone assumed WordPress was just for blogging (and you had to work pretty hard to do other things with it), WP has become more and more popular as a CMS.
There are still times when another CMS is going to be better for a particular job, but for most small business websites, including small web stores, forums, proprietary social networks, portfolio sites, podcast and video sites, and a host of other uses, WordPress is the quickest, easiest, most user-friendly option.
There are more than 50,000 free WordPress plugins to let you do cool things with your site—and some bang-up commercial plugins, too. You can choose from thousands of free themes (design templates), buy a spiffy premium theme, or have a unique theme custom-designed for you. The community of users is enormous, enthusiastic, and helpful, so you can find help easily on Twitter and LinkedIn as well as in the official forums and in local meetup groups. And there are plenty of consultants like me for hire.
Here are a few recent articles about the advantages of WordPress, in case you still need convincing.