I understand the differences between RAW and JPEG. But why does my DSLR offer me the opportunity to shoot RAW plus JPEG simultaneously. What benefits does this offer over RAW only, especially since I process my RAW files in Lightroom and the originally 'camera saved' jpegs would not reflect the changes. (I export jpegs for screen viewing after LR adjustments). What am I missing?
The only reason I can think of is quick proofing in a professional context. Since RAW doesn't apply the image processing directly, it can be handy to have a set of JPEGs you can dump from the camera and display to a client immediately. They can then pick what images they want and the RAWs can be used for the final product.
If you want to make use of things like Pictbridge, you also often need a JPEG for it to work directly from the camera.
So fundamentally, it is a little used feature (since the drawbacks, like reduced burst capacity and increased data consumption, rarely make it worth it) it does have some niche uses that can make it a nice to have. It's also a little bit more convenient if you don't plan to process all your images.
Personally I always shoot RAW only though.
I do it most of the time in my Fuji X-10 because the JPEGs are usually very good (the camera has the exact lens profile to apply all the necessary corrections), and if I need some more latitude or colour depth for a certain shot I can always pull the RAW (and do my best to correct the geometry in post processing).
For personal use and professional use where I have the time and the ability to post process my pictures I shoot RAW/Adobe RGB/Custom White Balance only. But I also shoot many events where the event wants JPG/sRGB files immediately, so in those cases I shoot RAW+JPEG.
With the Canon 5D I have the camera save the RAW files to the CF slot and JPG to the SD slot making managing the files much easier.
One of the major reasons I shoot both is because of the reduced file size of jpegs.... If a photo is well exposed and I'm not planning on doing much processing, I'd rather store the jpeg which is a 1/4 of the size. My jpegs are usually around 5-7MBs and I've seen raw files from my T4i up to about 29MBs. Since I backup to 4 different sources and have multiple weeks/months of backups on each source, it adds up fairly quickly. Even though hdd space is fairly cheap now if you add in the videos I also take and backup we are talking about some $.