If you need any of the Server services such as DNS, DHCP or others and have no second machine to run these on, this could be a reason to use the server version.
Server 2012R2 does have the modern tile interface, but it doesn't come with any of the Windows Store Apps that Windows 8.x comes with. It doesn't even have a store tile. I think you can get these features after adding the "Desktop experience" feature. But if you don't like these apps, Windows server hides them all.
Also Windows 8.1 makes it harder to use a local account and wants you to use a Microsoft-account to log on to your machine. By default server does not use Microsoft accounts.
2012 R2 still comes with the full Windows 7 backup feature, while 8.1 has removed most of this feature. If you need it, server is the better choice.
Previously a superior IIS version and Hyper-V were reasons to use server, but these are now just as good on the workstation OS.
Having said all that, Windows 8 is optimized for desktop applications while server has been tweaked to run background applications such as Windows Services. So for a desktop PC, Windows Pro is the much better choice unless you have a real good reason to use server.