Sooo what do I do with the "server" part of our newest addition?
Bear in mind that although, once upon a time, the server version of OS X cost several hundred bucks more than regular OS X, these days its a $20 "upgrade" from the App Store and largely consists of a "wizard" app for configuring facilities that are already available in regular OS X with a bit of command-line-fu.
For your sort of set-up the most useful thing you could do with the "server" software is to set up shared Time Machine backups and other shared folders for common files used across all of your Macs. However, it sounds as if your NAS already has that covered.
You could set up a multi-user mail server, but, quite frankly, if you are on a domestic internet connection and can't arrange for direct delivery of email via SMTP, using gmail or iCloud makes more sense.
Likewise, you can run a webserver - but since many domestic internet connections (a) have a fairly slow upstream connection, (b) don't have a fixed internet address or (c) have T&Cs that forbid running servers the main use of this would be for a home "intranet" and/or as a tool for web development.
At worst, what you've effectively done is spent $180 on a second 1TB hard drive. You could almost certainly have done it more cheaply with a regular Mini and a third-party DIY kit if you didn't mind ripping open your brand new Mac, but I wouldn't loose sleep over it.
