Back in the day, there was a piece of software known as FrontPage. It was touted to be the easiest way to create a web page, so easy even your grandmother could use it. The premise was that if you could use your computer to create a document, you could create a web page. All the interesting pieces that made a site were invisible to the creator. They were incorporated into the software or based upon the server. (Which was really nice since I worked for a company that made sure these extensions worked on the most popular servers.)
So jump forward to today. There are a number of software applications available to create your web site, many available on-line, hosted by such folks as Go-Daddy, Web.com, Yahoo, etc. With everything available to you why would want to use a design/development company? One of the best reasons is that if you hire a company then you can concentrate on your strong points and focus on your business.