A coaxial cable system works by capturing external RF energy in the outer braid and grounding it out before it interferes with the centre conductor, which is where the desired signal is carried. Therefore, to remain effective, the outer braid must be grounded.
This is the primary reason for grounding coax--even for an interior install.
In terms of the antenna, an indoor antenna does not absolutely require grounding. Though it's a good idea for discharging static, etc. Attics are often humid & hot places...where charge can build up easily.
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