Question: DSLRs often have the ability to store both a JPEG and a raw file.

Given that the [primary benefit of in-camera JPEG over raw](https://photo.stackexchange.com/questions/15/what-are-the-pros-and-cons-when-shooting-in-raw-vs-jpeg/115#115) is the smaller filesize, and that JPEG+raw is going to store even more data than raw alone, it seems like you're just wasting space on your card and [making your workflow more complicated](https://photo.stackexchange.com/q/4453/378) if you store both.

Why bother storing both JPEG and raw in camera, instead of just a raw file?

Answer: 
I am an amateur photographer going semi-pro and even though I still only use RAW I have come across a few occasions where RAW+JPEG was needed (or at least would be a great convenience):

* **ready to email files** (like @rowland-shaw wrote) - some times you need to get your photos out there as fast as possible
* ***lite* photo files to browse through** - given that your workflow might include taking a look in your photos from a not-so-capable computer (or other device) before importing them or even during the shoot, it is faster to load a 1.2MB JPEG than a 15MB RAW file
* **timelapse** - ok, this is an overkill but when shooting timelapse I want to have a bunch of small JPEGs ready to be opened in QuickTime to check the result and then go through the RAWs

In general, **JPEGs are for fast preview** on other devices (other than your camera) while **RAWs are for editing**.
I shoot JPEG + RAW because my camera produces *really good JPEG output*. It has flexible control over tone curves, color, and contrast. I'm not usually interested in producing HDR-compressed images — in fact, I often prefer a high contrast look which reduces dynamic range. If I get the exposure and other settings right, I really don't benefit much from RAW.

If I make a mistake with white balance or am in a tricky situation, I have the RAW file to take advantage of. Most of the time, I develop that in-camera, using the built-in tools to do so, but in the cases where I'm not satisfied with that, I use RawTherapee. (My camera allows adjustment to the color of the LCD; it's not completely color profiled, but it's basically neutral, so I can trust my eye well enough.)

I know that some cameras only allow highly compressed "Basic" JPEG in combination with RAW; mine lets me save JPEGs of any quality, and in fact, I usually use ★★★, only increasing to ★★★★ when the scene needs it or when the image appears particularly special. (See [Is it worth using the Premium JPEG quality setting?](https://photo.stackexchange.com/questions/21690/pentax-k-5-is-it-worth-using-the-premium-jpeg-quality-setting))

And, in fact, in the interest of keeping my lifetime data load sane, I only keep the RAW files for those particularly special images. I know this is verging on sacrilege against the conventional wisdom, but I haven't regretted it yet. If I had *paying customers*, I'd definitely archive it all, just in case.