What is Film-grain and why do you need it? What's the difference between Film-grain and Noise?
Let's talk about Film-grain first.
Film grain is an optical texture of photographic film caused by small particles which are made of metallic silver, or dye clouds that have received enough photons. Film-grain is an optical effect, the magnitude of which depends on both the film stock and the definition at which it is observed (Wikipedia).
Film grain Overlay
Film grain overlay, (FGO), is a process in which film characteristic elements are overlaid onto a digital file. This process adds film noise characteristics, subtle flicker to the more sterile looking digital medium. There are computer plug-ins that simulate Film grain.
Overlay vs Plug-in
Real Film grain occurs to be more present on bright areas of the image. While Fusion's Filmgrain Tool takes that into account, a noise/grain plate derived from actual film, is not dynamically applied. It means you need to be careful not to make the grain too present in the dark areas.
Film grain plates are shot against a gray card because when using the Overlay blend mode, the grey will not be affect to the image.
Grain vs. Noise
Do not confuse those ( as I did ) . Grain is the noise you get on real film while noise is the noise you get on digital camera. The difference is not only the granularity, but also unlike Film grain, digital noise appears in low-light situations or in other words, in dark areas of the image.
Noise in Video processing and VFX compositing.
Although Film grain looks nice and gives your digital footage a charming analog characteristic, it is an optical effect that should be removed before doing color works or compositing. Now you might be confused as why you should remove that nice effect. Well, you are only removing it temporarily. You can for instance generate a noise finger print which will give you a de-noised version and an extracted grain plate. Then once you are done with your compositing, you would apply that film grain back. The reason why it needs to be done is to avoid altering the grain /noise because it can affect pixel accuracy. Currently the best noise removing plug-in is NeatVideo. It's pricey, because it is really darn good!
Noise Extra Power
Noise or Film grain can also be used to remove color banding. Try it out. In Fusion you can use the Filter node, where you set the operation to noise. You notice there is an animate checkbox. Be careful not to neglect this option.
Film-grain from HolyGrain
I use Film grain all the time. One particular grain plate I like is one that the site HolyGrain offers. They have 16mm and 35mm film grain scans. They also have free ones which you can test. You can really tell the difference between real grain and artificial grain. So for people using digital film I definitely recommend this over the Fusion film grain tool.