OS X Server is basicly an additional app that gets installed on top of OS X. The server basicly gives you a controll panel for diffrent server related services. My MacBook Pro from 2010 have this software installed and I use it for experimenting with websites inside my LAN. The only reason for installing the server software on a personal mac is for testing purpose.
Only if you want one of the features server provides. For example, some people run server so they can backup multiple Macs over their local network to a Time Machine backup disk attached to the main, server Mac.
But no... if you don't need any of those features, it does not do anything extra for you.
There's no need since its the same OS as mentioned. I was tempted to do this years ago, only to find out that I'm not getting anything different (other then some apps).