I saw an interesting explanation about the purpose of this mission from Neil deGrasse Tyson. There is a very small chance that the Asteroid Bennu could hit the Earth in approximately 180 years. One of the purposes of the mission is to understand the structural composition of the asteroid so that we know what we would need to do in case the chances of a collision with Earth increase and we need to do something about it. That way we have a better chance of deflecting it instead of just scattering it and creating a larger number of impacts.
He also explained that the size of the impact would not be an extinction event, but depending on where it hits it can wipe our an area the size of a large city (if it hits on land) or could cause a giant Tsunami that could race across an entire continent if it lands in the ocean.