You should log out properly so that another user cannot continue your session. If you turn off your terminal or hang up your phone when you have a dial-up connection, the system may not be able to disconnect you and kill your shell before another user is connected to the same port. This new user may be connected to the shell session you thought you were terminating. If you are using a hard-wired terminal, you may not be logged off even if you turn off the terminal.
You should log out using either exit or CTRL-D. When the system responds with
login:
you know that your session has been properly terminated.