Also, if you dont shoot raw all the time you can choose both option, use your jpeg and if you have a picture you feel you want to work on with further processing you have the original raw file to do so.
Or if you have an ipad or other tablet, you can download draft versions of the shots (jpgs) onsite before you had a chance to work on the RAWs. It is great for quick proofing.
I usually set jpeg to small size and also use them as thumbnails in the same folder on my computer to identify whch RAW to work on.
Many other reasons... e.g. picture styles work on jpegs, as do multishots and HDR.