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Because you don't want to be watching a movie that is at a good sound level and then have a commercial come on that blasts your eardrums out.
Automatic Gain Control controls the volume of RF & IF frequencies to a steady volume. SInce most commercials, movies, shows, radio stations project their sound different something was needed to mitigate the rise and fall of the volume, hence the AGC.
In reference to RF v.s. IF. The AGC needs to be abe to work with these on a TV because now-a-days you can get radio on your television. Different stations have different sound output and the AGC manages that and projects a steady volum. IF is for anolog television. IF includes three phases but the AGC only affects the first stage. The downconvert stage. (the other two refer to b&w and color)
So the next time your at someone's house and they are complaining that their commercial are too loud compared to the show. Have them look in replacing their Automatic Gain Control. ;)
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