Another use for an ethernet card might be to connect to a netatalk server. Your G4 with 10.4 might be able to do this too if it were on your local network, but more or less, your macbook and your LC III would both be able to connect to the same file server and you'd be able to just leave files on the server if you wanted to.

If you don't have a spare computer, you might try out Ubuntu or Debian in a virtual machine, such as with virtualbox or the vmware player. You'd just need to install netatalk (apt-get install netatalk) and then it should show up in the chooser on the LC III when you have the ethernet card installed and hooked up.