The electron beam needs to get back to its starting point.
Blanking
The intensity of the electron beam during the scanning process is not constant. It varies in a logical, consistent pattern corresponding with the brightness of the picture as the beam moves across and up and down the CRT's face. If the electron beam were at full power when it returned to the beginning of a new line or field, it would illuminate phosphors on the CRT face and interfere with the information previously laid down. In order to prevent this from happening, the electron beam is turned down to a very low power so that it can return to the beginning of a new line or field. This period of time when the electron beam is turned down is called blanking.
Horizontal Blanking ...
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