I manage the network for a small office. The office has 1 server and 5 client machines in a Windows domain. The old server is running Windows Server 2012 Standard and the client machines are running Windows 8.1. I've purchased a new server and installed Windows Server 2012 R2 on it and joined it to the domain, although I haven't defined a role for it yet.
I see a lot of people using Windows Server as a client OS. For example they use Windows Server 2008 R2 instead of Windows 7, Windows Server 2012 instead of Windows 8, Windows Server 2003 R2 instead of Windows XP.