3ds Max Modifiers - Mirror
The Mirror modifier provides a parametric method of mirroring an object or a sub-object selection. You can apply the Mirror modifier to any type of geometry, and you can animate the mirror effect by animating the modifier's gizmo.
My 2 Cents : Mirror modifier allows for procedural mirroring of objects. As a one time operation I do not find to be that great (I'd rather use symmetry modifier). That being said, if stacked, the mirror modifier can then be used as almost a procedural radial array. Like most modifiers, Mirror can be applied to sub-objects which again takes advantage of 3ds Max's procedural and/or parametric modeling capabilities.