Besides advantages (using 3dconnexion space navigator) that were already explained above by others, I want to add other two:
Meanwhile building or editing (w/ the mouse), I navigate (w/ space navigator), especialy in large models: simultaneous actions.
‘Space navigator’ is much more useful than the mouse for screen capturing (using a program such as Camtasia Studio) a walk-thruogh or fly-by animation.
Cornel