Gamma Correction is used to adjust the picture brightness to look
natural on the screen. This type of correction can also be needed if
the video was recorded under bad weather conditions or if the camera
produces too bright or too dark results.
Gamma Correction is a non-linear function that produces different results
from Brightness or Adjust RGB.
Values above 1.0 will make the picture brighter, while values below
1.0 will make it darker.
However, completely white or black pixels are never modified.
Another use of Gamma Correction is to enhance contrast. Say you have a badly scanned text with grayish
letters on the white background. By applying gamma of 0.5 paper color
will stay the same (if it was pure white) while grayish letters will get darker, maybe even black.
This will make text more readable.