Often you want to test out some simple static http / css / js code in your local environment. You could setup mass virtual hosting and dnsmasq with a wildcard domain to serve *.dev, but for purely static files, I find this is an easier setup. And there is less overhead, as pow spins up nodejs to serve a request rather than constantly having apache run in the background.
Pow is a zero-configuration Rack server for Mac OS X. You can install it with homebrew
brew install pow