Q: How is this different from just adding a strut brace or sway bar?

A sway bar or brace reduces body roll and flex, but neither addresses the compliance built into the rear suspension links themselves. These arms attack the source: the rubber bushings that let the wheel wander under load. Fit them alongside the adjustable front and rear sway bar end links and you get both stiffer roll control and a rear end that holds its geometry. They solve different problems, so they complement rather than replace each other.