Past methods of exoplanet detection relied on the transit of a planet in front of it's parent star and watching for a dip in the light from that star. This method would tell you the size of the planet, but not the mass. The new method will help determine the mass of planets through the gravitational pull they have on their parent star.
This new method will help determine if an exoplanet is a rocky world like the earth, or a less dense gas giant like Jupiter.
Source (The Optical Society)