There are many good reasons here, but here are some other reasons I can think of.
- JPEG is a standardized format. Most RAW files are not. Programs rely on the RAW profiles to be installed to process them and allow you to work with them.
There is DNG and TIFF/EP that aim to standardize RAW files, but very few cameras have adopted these.
If for whatever reason the RAW files aren't supported in future software, maybe because the cameras at that time is considered obsolete, you will at least have a JPEG image until you can obtain those RAW profiles.
- JPEG is a lossy format, meaning data is lost in the compression process. If you primarily work with JPEG files and have the means/space to store RAW files it would be a good idea to do so, even if you don't use them much. RAW files are lossless, so you can always go back to the RAW file and get all the RAW data from the camera and start over, if needed. If you shoot only JPEG then you don't have that option.
Saving both does use a lot a disk space. There are other options, however, like online services, DVDs, tape (if for some reason you have one). If you archive your RAW files it's best to have at least two copies and one off-site so you don't loose your RAW files if your hard drive or DVD holding them dies.
- If you like how a shot looks on your camera it's easier to compare and recreate processing of RAW files on your computer. RAW files don't always save the JPEG processing done on the camera, except maybe in a very small preview image, so RAW files end up looking rather neutral / bland after they are first imported.
Other Notes: If you want to keep both files and are worried about camera support in the future you can always convert the RAW files to DNG files. The idea behind DNG is that it's standardized, so companies like Adobe will continue to support the format 'forever'.
If you use a tool like Lightroom you can automatically have JPEGs, PNG, DNG, or any other format automatically generated with whatever adjustment/processing profiles you like. This saves space on your memory card, but it doesn't take into advantage of the develop processing on your camera. That's done in Lightroom at that time.