Boring the block squares the bore, cuts out scores, and evens out the diameter of the cylinder bore.
I pulled a 283 years ago, that had one cylinder where the piston had worn down on one side only. The cylinder was scored, and almost oval in shape. The machine shop used that cylinder as the prime cylinder, and when it cut square, machined the other cylinders to match it.
Boring, aside from making the cylinder diameter larger, also removes any irregularities time has done to the cylinder. From this point, cylinder honing smoothes out the cylinder wall, and induces a cross hatch pattern to seat the rings.
I have seen boring jobs, where the entire cylinder bore was bored out, the remaining material threaded and a cylinder liner installed. I have also seen articles of boring out the cylinders, and pressing liners in. In this application, the center cylinders were welded together.