Calculating the orbits of asteroids is indeed not accurate enough to calculate the chance of impact, but it is possible to rule out an impact. The majority of the thousands of asteroids found today are not harmless. Most of them won't cross earth's orbit in near (= hundreds of years) future, will leave the solar system or are small enough to burn in earth's atmosphere. There are, however, potentially hazardouds asteroids for which collision can't be ruled out. Odds are that they won't collide, but there may be a small chance (e.g. 0.2 % chance of impact in 20 years). "Pushing" these asteroids a couple of meters to the left now, results in a different orbit which reduces or completely takes away the impact risk a couple of decades later. That's the idea.
Diverting asteroids this way won't keep us safe, because the real danger comes from asteroids we have not discovered yet (so we don't have orbit-data), comets and other objects coming from the direction of the sun.