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That option will often cause a loss of quality. I would suggest avoiding it for final exports, unless you know one of the few situations where it is safe.
Steve, could you explain why there would be a loss in quality? If that is the case, what is the purpose of having the option to use the Previews?
Many render codecs use some form of compression. During export, you are typically adding another layer of compression. Combine this with the original compression caused by the camera, and the deleterious effects of compression start to add up. The idea is to avoid any compression step whenever possible. Unchecking that box uses the original file, not the compressed preview, thus eliminating one layer of compression from the production chain.
> If that is the case, what is the purpose of having the option to use the Previews?
If you're creating a movie to show a client for a review of your approach to an edit, for example, then you needn't worry quite so much about the quality of the images, so you can save on time.