As Rob stated earlier, if you have JPG, you've already lost about 30% of the information in the file. Now, do any PP of any sort, even resizing, and save that as a JPG and you've lost even more of the information/quality.
A RAW file gives you the base from which you can do any PP from and as long as you keep the original file you can come back to it time & time again. Every time you edit a JPG & then save it as a JPG, you lose quality.
So, if you only need a 800 pixel image for the web, JPG should be fine, if you want a 24 x 20 print, then personally, I wouldn't be using a JPG, I'd edit the RAW file, and save as a TIFF.
I have one portrait that I took 2 years ago, probably one of the best I've ever taken. I have edited that image probably 20 times as I use it as a "base" to learn new techniques. Each time I start from the original RAW file, that way I get consistency.
Steve