The built in server reduces the number of steps to developing a php web app. A lot of devs I know go and download MAMP/XAMP because it's the easiest way to get a running web server. But those packages are a pain to work with and are generally a couple versions behind. The built in server allows you an easy way to use the latest version of php, while not having to worry about apache. Devs like to code - not setup apache instances..