NetworkManager attempts to keep an active network connection available at all
times.  The point of NetworkManager is to make networking configuration and
setup as painless and automatic as possible.  NetworkManager is intended to
replace default route, replace other routes, set IP addresses, and in general
configure networking as NM sees fit (with the possibility of manual override as
necessary).  In effect, the goal of NetworkManager is to make networking Just
Work with a minimum of user hassle, but still allow customization and a high
level of manual network control.

You will probably want to install network-manager-applet (for a configuration
UI) for sure, along with ModemManager (if you want support for configuring
broadband modem connections) perhaps one or more of the VPN-related addons.

You will also need to start rc.networkmanager during boot - you can use
rc.local, or perhaps you'll want to edit rc.M's call to rc.wicd...  :-)