Digital sensors are clean. Modern cameras can shoot in near darkness and produce files with very little noise. So why do so many photographers pay for presets that add grain back in?
Because clean can look cold. Real film grain comes from tiny silver crystals in the emulsion, and their random pattern gives an image texture you can almost feel. Digital noise is different. It comes from the sensor and the electronics, and it tends to look blotchy and colored rather than fine and even.
What grain actually does
Grain hides things. Smooth gradients in a sky or a wall can show ugly bands after heavy editing or compression, and a little grain breaks those bands up. It also makes an image feel less perfect, which some subjects need. Street photography and documentary work often look wrong when they’re too polished.
How to do it well
Keep it subtle. If you can spot the grain at a glance, you’ve probably overdone it. Add it last, after sharpening and resizing, and check the result at the size people will really see. Grain that looks lovely at full size can turn to mush in a thumbnail.
And if you like the look, shoot some real film. It costs more per frame, which makes you think.