In a smaller tube it's not such a big deal. But in a 30 or 35 inch diagonal measure tube the angle that the electron beam must make to reach the corner is extreme. And as a result the beam of light produced on the faceplate is distorted - it's not the same as in the center. The end result of this is a fuzzier picture at the corners, and perhaps inaccurate color as well. So the engineers got busy and designed special circuits that take account of the angle and make adjustments when the beams are approaching the edged of the picture tube.
Pictures are uniformly sharp across the entire tube, and color are accurate also.
Of course not all larger TVs contain this circuit. Those that do sell for more, but also deliver a better picture. And the same thing is true for the other circuits we mentioned - velocity scan modulation and dynamic black level. This provide better pictures - but they're not found on all larger tube TVs. So when you're looking for a larger tube TV, take your time.