You should consider premultiplying an image if you have to modify a foreground element and composite over a background image. Furthermore, you should also consider premultiplying two or more images whenever you have to perform a color correction or use filtering nodes.
When you premultiply an image by a matte, you should be aware that there is a very specific brightness relationship between the pixels in the color channels and the pixels in the matte. Since certain systems that will assume that you are working with unpremultiplied systems, will also assume this brightness relationship. As such, the brightness of any color channel cannot be modified without also impacting the alpha channel.
There are two general rules to remember when considering premultiplication:
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