Network Manager aims for Network Connectivity which "Just Works". The computer should use the wired network connection when it's plugged in, but automatically switch to a wireless connection when the user unplugs it and walks away from the desk. Likewise, when the user plugs the computer back in, the computer should switch back to the wired connection. The user should, most times, not even notice that their connection has been managed for them; they should simply see uninterrupted network connectivity.
https://help.ubuntu.com/community/NetworkManager0.7
http://wiki.debian.org/NetworkManager