Professional photographers shoot so many RAW files they would quickly fill a computers internal hard drive. For this reason they will frequently keep all of their RAW files on external hard drives within nicely arranged Folder structures. When they import these RAW files into Lightroom (using Add, not Copy), Lightroom will reference these external images. Any edits that are made to these RAW files are stored within Lightroom along with a good quality preview image. It is possible to push this editing information to the individual RAW files (technically a sidecar file) but typically this is not done. The purpose of a good quality and locally stored preview image is so that Lightroom has something to display even when the external images are unavailable (eg external drive not connected). JPEG images can be exported out at any time and are made by applying the locally stored editing information to the externally stored RAW files. With all this in mind we can see that a RAW file can be "re-edited" as many times as desired without altering or damaging the original RAW file. Personally I like to process single RAW images in multiple styles. When I do this I don't want to mess up a previous look that I am happy with. This is why I use Virtual Copies which allow you to copy and individually process a single RAW file as many times as you like. These Virtual Copies only exist within Lightroom and all reference a single and unaltered RAW file. Hope this helps.
... View more