OS X Server has just been one huge experiment for Apple.
Mac OS X Server 1.0, where it all started, simply was the commercialised produce of the NeXTSTEP -> OPENSTEP -> Rhapsody process after Apple bought NeXT.
Once Rhapsody / Mac OS X Server 1.0 got the Carbon API and Aqua GUI it was all about Mac OS X (10.x) as a client OS.
As the early OS X client releases only supported NetInfo as a directory service (a NeXT legacy) it was necessary to be able to host a NetInfo database over the network to serve Macs on the corporate LAN.
Mac OS X Server was the logical product to host such a service. There were enough Apple System Administrators who were willing to take the plunge in what was called Mac OS X Server.
As OS X already had all the "server" features (FileSharing, DHCP, DNS, NetInfo / LDAP etc.) built in by default (as it is a UNIX) it wasn't a huge big deal for Apple to make a GUI which could administer these services.
So, Mac OS X Server 10.0.3 was born. At first this GUI front-end was based on the (absolutely awful-) AppleShare IP interface. I assume this was to "help" the Mac OS 8/9 System Admins get used to OS X Server...
The Xserve G4 was released as a "Power Mac G4 on its side" but with Mac OS X Server 10.1.x as pre-installed OS. No real redundancy at the time, as there were no dual-power supplies back then. But, hey.... it did fit in the 19"rack.
Later the Xserve became a more sophisticated product, especially once the Intel Xeons were used. But, once you seem to get "serious" in the Server market, one new service becomes important: Support.
That seems to be a pain in the a**: Hardware support and software support....
For the "new" market called AV editing, Xserves combined with multiple Xserve RAIDs running Xsan and Power Macs (Mac Pros) all connected via Fibre Channel gave the Final Cut Studio specialists even more reason to use Apple based hardware and software jammed into 19" racks.
But both the hardware (newer, faster laptops with SSD) and the software (Final Cut Pro X) made this extremely expensive, sensitive, fragile and support-needing hardware and software redundant.
Nowadays an editor will use the software himself to AV-edit on a Retina MacBook Pro. No need for expensive 3rd party "experts" anymore.
Mac OS X 10.1, 10.2, 10.3 etc. came and went, and yes for each client OS version there was a fully-blown Server version.
NetInfo was replaced by LDAP v3 and later the OS X clients could even connect to AD databases.
Up to and until Mac OS X 10.6, the server versions were better and more feature-rich than the server versions before it.
Mac OS X 10.6 Server is probably the pinnacle of OS X Server.
Since Mac OS X Server 10.5 Apple has tried to give the non-techie-system-admins a simple interface to use and configure a server. At first this "kiddy-version" of Server Admin (Server Prefs) was neglected. It wasn't even compatible with the "normal" version of Server Admin.
In 10.6 Server the "geek" (Server Admin) and "kiddy" (Server app / prefs) versions were compatible. So, teh "kiddy-version"
could be used, but no-one did.
In 10.7 Server the "geek" version was not even installed by default. So, the geeks had to download and install Server Admin manually....
In 10.8 Server the "geek" version is GONE. Only the Server.app is there.
End of geek use, enter kiddy-only use.
The Xserve is long gone. No real need for 19" Apple hardware anymore.
"Real" servers were never based on Apple software or hardware. The occasional NetBoot / NetInstall can be done on a Mac mini. If you really need some sort of horsepower to run OS X Server, get a Mac Pro.
But.... guess were the Mac Pro is heading...
Apple, logically, did to the Xserve what Microsoft did to the Zune.
So, in retrospect (funny... "Retrospect".... that was the only really usable OS 8 / 9 / X Client and Server backup software...) the Xserve was a temporary need for AV purposes and for the need for early OS X client support.
Not anymore.
In the early says of OS X (10.0 - 10.4) it was kinda necessary to have a OS X Server running to help support Macs in a network.
Since iOS (iPhone OS to be precise) Apple has made sure their end-user products (the game Apple actually is in) connect and can make use of
existing Servers.
The iPhone was the first Apple product that connected better to MS Exchange than to other Mail servers, including Apple's own OS X Server Mail services.
IMHO Apple System Administration doesn't really exist anymore. OS X Server is just an app that helps the end-user setup some services on your home or small-business network. Yes, setting up a DeployStudio Server running DHCP, DNS, NetBoot, AFP etc. seems to need system-admin's knowledge and thinking, and that's true. But the level of support needed to keep those type of services running is very low....
IT is getting more and more "extreme" Either "simple" IT (DHCP, DNS, FireWall, Routing) is left to the end-user, or "complex" IT (Cloud hosting, hosted Exchange, etc.) is left to corporate professionals.
The need for the "man in the middle", i.e. the Sys Admin, the geek in the family, will fade away. New technology gets simpler to use, and, the modern users get more tech-savvy.