Objects such as 1996JA1 are hardly unique. The two main threats to Earth come from wayward comets and asteroids that have been bumped out of their routine paths into Earth-crossing orbits. In order to mount a credible defense against asteroids and comets we need to have a better understanding of their numbers and advanced warning of a potential strike. A well organized international search strategy is needed. Space based research missions such as the U.S. Air Force Clementine and NASA's Near-Earth Asteroid Rendezvous are also needed.
Most natural disasters are more limited in magnitude than a large impact.
A large asteroid or comet has the potential of knocking humankind back into the dark age, or worse. Large tsunamis could destroy coastal areas, global fires could burn out of control or seismic shocks could flatten large cities. Unlike the dinosaurs, humankind has the ability to prevent such a disaster.