Though we have gotten good at detecting near-earth objects in recent years, and are getting even better all the time, the fact remains that we are still discovering new objects, previously-unrecorded, whose trajectories bring them extremely close to our world. It's not inconceivable that, despite our best efforts and the advantages of modern technology, we might be unaware of potential threats to our planet until a relatively short amount of time before they impact. The most prudent course of action, therefore, is to have a plan ready to be enacted beforehand, in case such a scenario ever arises.