Yeah, it's not like you can't just sudo apt-get nginx.
But what if you want to run php or perl cgis? Not that simple anymore.
This makes it simple again.
This creates your web root under /var/www - it'll be obvious, I give you some silly default page and so on.
Saves a ton of time and typos. NGINX is the webserver of champions - far faster and simpler than Apache, which is why the really big boys use it these days.
For some systems you might have to go into the /etc/passwd file and edit the login for www-data to change it from "nologin" to /bin/bash. The NGINX user needs a shell to run some of the CGI stuff.