It's for manually changing the layout - swapping gates, adding/subtracting parts or connections, copying circuitry, etc.
Not something you want to mess with if you're inexperienced in board layout. Very easy to introduce errors or unwanted modifications/changes to your board.
One thing you may still need to use ECO mode for, even as a new user, is adding fiducials and mounting holes.
By the way, tfere are tutorials and other training aids on Supportnet. Very useful and helpful for new users.