Why does it change every time?

Because the specks are scattered at random, which is what grain is. Press Again until you get one you like, then download that one. The settings stay put.

Should I use the SVG or the PNG?

The SVG if you are putting it on a website, since it is smaller and stays sharp at any zoom. The PNG if you are taking it into a design tool or a video editor, where a raster overlay is easier to blend.

How do I use it as an overlay?

Leave the background empty so it exports with transparency, then lay it over your image and reduce the layer opacity until it looks right. Starting around 60 percent and coming down is usually about right.

Grain Generator

Film grain, by thickness and spacing.

Thickness

Distance apart

Colour

Opacity

Size

Background

Film grain, by thickness and spacing.

How to use it

Set thickness and distance. Thickness is how big each speck is. Distance is the average gap between them.
Pick a colour and opacity. Leave the background empty for a transparent overlay.
Download SVG or PNG. Press Again first if you want a different scatter.

When you would use this

Film grain is one of the quickest ways to stop a flat design looking flat, and generating it is mostly a matter of exposing controls that mean something. This one takes a thickness and a distance, because those are the two things you can actually see. Thickness is how large each speck is, distance is the average gap between neighbours, and the number of specks follows from the area rather than being a control of its own. Nobody thinks in grain counts. The alternative approach uses a turbulence filter, which produces a frequency number that means nothing until you have tried six values. That version is worth having too and it is the texture generator, which is a separate page because it is a genuinely different thing: a continuous noise field rather than scattered particles. Leave the background empty and the export is transparent, which is what you want for an overlay. The SVG is the better choice on a website and the PNG is easier to blend in a design tool, so both are offered.

Questions

Why does it change every time?: Because the specks are scattered at random, which is what grain is. Press Again until you get one you like, then download that one. The settings stay put.
Should I use the SVG or the PNG?: The SVG if you are putting it on a website, since it is smaller and stays sharp at any zoom. The PNG if you are taking it into a design tool or a video editor, where a raster overlay is easier to blend.
How do I use it as an overlay?: Leave the background empty so it exports with transparency, then lay it over your image and reduce the layer opacity until it looks right. Starting around 60 percent and coming down is usually about right.