Add grain for texture and to hide banding in gradients — it gives footage a more organic, film-like quality.
Steps to add noise and grain
Apply Effect → Noise & Grain → Add Grain or Noise depending on the look you want.

Increase intensity until you see a subtle texture that feels right.

Preview playback — grain can be heavy on render times, so test it before you commit to final export.
One more thing
Apply grain on an Adjustment Layer at low opacity for a lighter, film-like effect instead of applying it directly to layers.
Related how-tos
How to add black and white filter in After Effects
How to create a vignette effect in After Effects
How to sharpen video in After Effects
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