Win 2000 Cal licence

Silly question, I think it is, but how does the win 2000 Server 5 Cal work? I really thought that I could put win 2000 on 4 other computers but I don’t really know. Always used a peer to peer before. I guess that I can use the server and have 4 other systems using it as a file and application server. Otherwise, I can’t see the benefits. I have 5 systems in a small office network. Ta.