It used to be that a Mac server was something only a systems administrator would ever want to go near. It was a special, expensive version of Mac OS X — for example, Snow Leopard Server cost $500, as opposed to $129 for the regular version of Mac OS X. And the updates had to be supplied separately, and the tools were hard to use.
Things have changed dramatically. In Mountain Lion, OS X Server is just an app, in the Mac App Store, that costs $20. You can literally just install add it on to any Mac, and administer it with dramatically simplified controls.
Why would you want to, though? Well, if you want to share files on your network with other Mac users, you can do so with an unlimited number of users, and much finer grained controls.
You can also set up profiles. These allow you to “push” settings to iPhones, iPads, and Macs in your organization. For example, if you want to be able to have someone access your company email account, you can just have them visit a web page on their phone to “enroll” with the server, and once that’s done, you can configure their email without having to even touch their phone. And, if they leave your company, you can remotely remove the profile, taking your email with it but not touching the rest of the phone. There’s much, much more you can do with profiles to remotely manage and control devices and Macs.
There’s a bunch of other stuff that OS X Server does too, but these are two of the big ones. For $20, it’s a pretty good deal.