It's often the case that it's not known how many samples are needed to render a picture. So I usually set a higher value and when the result already suits me I stop rendering. But if you applied compositing to picture, it doesn't apply when you stop rendering. So how do so that compositing apply when I want to stop rendering before it reached max samples?
You can save the result of progressive rendering as an image file (exr for maximum information) then do the compositing in an external .blend-file. See also: What advantages in using progressive rendering?