I think the paragraph about naming the hostname is entirely clear. If you bind a socket server on localhost, you can indeed only access it from your own machine. If you want the server to be accessible from any interface (wireless, wired, local, whatever), just specify:
php -S 0.0.0.0:8080
As for the 'what's the benefit' questions.. for simply stuff it now becomes extremely easy for library / application developers to just include a './launch.sh' script to get an application running.
This makes it very easy for people to evaluate software, without the user having to go through documentation that describes which php.ini settings need to be set, and webserver-specific configuration. Just 'run and go'.
Examples of other applications I've seen that do this is mercurial (hg serve) and gollum. I don't know much about either Python or Ruby, so I was happy I could just fire up the command line and be up and running.
I suspect that we'll see a lot of applications that will start supporting this.