Already described in the architecture document:

https://github.com/ros2/launch/blob/88ae20ee2e950df91e5594975a82bffe1ebd928b/launch/doc/source/architecture.rst#basic-actions

Basically it will allow you to include another launch description, which is given to the constructor of the new action. You can already do this by emitting an event, launch.events.IncludeLaunchDescription (https://github.com/ros2/launch/blob/b78d87f31bb995c7226df66826e1a57079ae3a3f/launch/launch/events/include_launch_description.py), so at first this action will be only syntactic sugar but later it should interact with launch arguments (see: #107).