Because if you don't reclaim the energy of matter infalling at 0.12c your craft gets shredded by dust. At that velocity it's like flying through sandpaper. And

Because if you don't reclaim the energy of matter infalling at 0.12c your craft gets shredded by dust. At that velocity it's like flying through sandpaper. And if you're flying through a cometary halo, your odds of hitting something bigger are much higher.