From the class definition:
Manages a matinee scene. Contains a list of action items that will be played out in order.
SceneManager is the class which controls the action(s) of the player's viewpoint (the "camera") and Actors (usually Movers??) during cinematic scenes, including motion through the scene, event triggering, fades in and out between scenes, and a number of other properties.
SceneManager (and Matinee) need not be used only in "cutscene" type cinematics. It's also useful in normal gameplay scenarios.
Matinee is the GUI used in UEd 3 to create SceneManager content (Properties and Action items) and to define motion paths.