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**.
Depending on your camera, there might be a good reason to shoot JPEG + RAW even if your workflow is RAW-only: **accurate on-camera previews**.

Some cameras work like this (IIRC, I have seen this behaviour at least on Canon PowerShot S95):

* If you shoot RAW-only, the camera will store a *low-resolution* preview JPEG inside the RAW file. If you preview images on your camera, it is only able to show the low-resolution JPEG. If you zoom in to make sure it was properly focused, you will always see blurry pictures.
* However, if you shoot RAW+JPEG, the camera will use the *high-resolution* JPEG file for previews. This way you can actually use your camera to check if the focus was correct or not.

Now you can choose between two options: a bit more space on your memory card (RAW) vs. accurate on-camera previews (RAW+JPEG).

With Canon DSLRs you do not have the same issue, as the preview JPEGs that are stored in the RAW files are of a high enough resolution.