GNU Screen is a useful program that allows a user to keep processes running on a machine between logins. It's great for long-running tasks like compiling a large program or running a lengthy calculation. The user can start a Screen session, start the process, detach from the session, and then reattach later, potentially SSH'ing in from another device.
Screen is also useful for lightweight or rarely-performed tasks. For convenience, Screen sessions can be labeled on creation with the -S flag. A user might run a chat client like WeeChat, reattaching to his "chat" session when he or she wants to reconnect with friends. The NFS setup on one of my Raspberry Pi's requires occasional tweaking, so I keep an "nfs" Screen session running. I can then reattach when needed and use the up arrow key to quickly retrieve and rerun the needed commands.