You generally Install the Server Software and configure one extra computer (not one for anyone working) to be the Server Computer. Then connect them all with Gigabit Ethernet and you can share files and provide services to all the other computers for no additional charge. Many Users run their Server computer without a display attached.
In the old scheme, there was a blob of Server software, and a separate Server Admin application, so it was easier to see the distinction. You installed the blob of Server software on the Server computer, then use Server admin to set it up and maintain it from any Mac on your Network.
But today all the Server Software has been hidden inside Server.app, and you use that to both create new Servers and to Administer your existing Server from any Mac on the Network.