This number represents the distance between the two bead seat areas where the tire is sealed onto the wheel. The wheel size also includes its width and offset. The width is the distance from each bead seat (from inside to the outside).
Larger wheels offer better traction, and because they have more rubber on the tire, this also means a better grip on the road. Larger tires are better for car cornering and handling than your regular tires. They also reduce the braking distance and improve braking overall, adding to safety.