How to use cat with socket?

While reading the source code of cat command, I found cat command supports for reading from socket. You can view the source at http://src.gnu-darwin.org/src/bin/cat/cat.c.html. But I've never use this command with socket: just quickly viewing a file or concatenating multiple files. What can I do with cat + socket? Can you give an interesting example of using cat command reading from a socket? Thanks.