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.