Normally this is used to start nodes based on a launch file.
Normal Usage:
roslaunch <PACKAGE> <LAUNCH_FILE>
This will list the nodes that a launch file will run on stdout.
roslaunch <PACKAGE> <LAUNCH_FILE> --nodes
This allows for changing the topic that a node publishes or subscribes to. This is really useful when using image_view.
roslaunch <PACKAGE> <LAUNCH_FILE> <ARG>:=<NEW_VALUE> <TOPIC>:=<NEW_TOPIC>