I took a bunch of before and after pictures to show the benefits of DC restoration. The video source is a 1992 SNES that outputs a 240p video signal.
Top is before, bottom is after. Picture has high average luminosity, so without DC restoration, the brightness is too low.
Top is before, bottom is after. Picture has low average luminosity, so without DC restoration, the brightness is too high.
Left is before, right is after. This is an extreme close up of a 240p test pattern of 240 horizontal alternating white and black lines. The ideal picture is for the black lines should to be thicker than the white lines, since this is 240p video where every other line of video is supposed to be blank, and so each black line is displayed next to a blank line in the 240p video that is displayed on a 480i TV. It is a great test pattern for measuring cathode ray spot size (beam diameter), which is useful for tuning the electron gun for a maximally sharp image.
DC restoration helps keep the peak cathode voltage very consistent, which is crucial for making the cathode ray as sharp a possible. Here I set peak CRT cathode voltage, which the DC restoration mod clamps to, to be 250 volts (near max). CRT G1 anodes are 175 volts (near min). CRT G2 anodes are 670 volts (max). Focus anode is 270 volts.
Not sure why this is not talked about more openly in vintage TV discussions, but cathode ray beam sharpness is highest when cathode to G1 voltage is as high as possible. This means lowering the TV's "brightness" as low as it can go (this increases cathode to G1 voltage), so that the picture is completely black, and then achieving correct black levels or cutoff by increasing the G2 screen anode voltage. Many TVs will hit their max screen voltage, and brightness will be too low. So the "brightness" setting, (which adjusts the cathode voltage to G1 voltage) needs to be slightly increased to achieve correct black levels. Maximizing cathode to G1 voltage squeezes the cathode ray to a sharp point in the beam forming region that is in between the G1 and G2 anodes, but this also means that G3 screen voltage must be maximized to prevent the picture from being too dark.
Since this technique relies on setting the peak cathode voltage to a very high value, it only works correctly if DC restoration is in place. Once the beam is sharpened this way, then the focus voltage should be adjusted to further sharpen the beam. The reason why you should only tune focus voltage AFTER tuning K, G1, and G2 voltage is because changing the focus voltage changes the focusing power of the lens... and changing the focusing power of a lens impacts two aspects of the electron gun "optics": (i) focal length, and (ii) magnitude of spherical aberration.
Spherical aberration is never a good thing because it causes the electron beam to scatter, even when the lens has been tuned to have the correct focal length to the phosphor CRT face. So when you are tuning the focus voltage, you have to balance focal distance with the amount of spherical aberration to get the sharpest image. The thinner the beam is, when it enters the lens, the less spherical aberration occurs. So sharpening the beam in the beam forming region by maximizing K to G1 voltage means that the focus lens can potentially achieve a better trade off between spherical aberration and focal length
Top is before, bottom is after. Here is a comparison of the improved sharpness. Keep in mind that this is a first generation Portacolor that uses a low voltage focus CRT with a gigantic dot pitch.