Programs like LightZone, Darktable, or RawTherapee — or proprietary software like Adobe Lightroom — have RAW conversion as a centerpiece function, but they're more than that.

They're also (to greater or lesser degree depending on the program) photo library management tools, with functions for sorting, labeling, and publishing images. This aspect can generally work on JPEG files as well as RAW originals.

And, in addition to simply loading and converting RAW images, they all have editing functionality. These editing functions can usually also be used on JPEG files, with some limitations inherent to not starting with a much data as RAW provides. (See "the value of RAW" answer to Good examples of RAW's advantages over JPEG? for details on what exactly that means.)

There are two reasons you might do this:

First, these programs are focused on photography, while general-purpose image editors aren't.

For example, on Linux, Gimp is the premiere image manipulation program, and its feature development recently aims for graphic design and image creation as primary tasks. So, it's nice to use a tool which actually is centered around photographs. See What are the key photography-related features from Photoshop that are missing in GIMP? for some things Gimp doesn't do — many of these aren't missing from LightZone, Darktable, or RawTherapee.

Second, and in general rather than just on Linux, these tools have non-destructive editing as a fundamental design — the "edits" you make don't change the actual image file, but are recorded as a series of steps to apply. This can actually be particularly good for JPEG files, because if you (for example) spend some time doing photo touchup, save the image, and then later realize you also wanted to do some color correction, your normal choices are a) do the touchup over again or b) accept extra quality loss from recompressing the JPEG a third time. With a non-destructive editor, you simply add in the new adjustments and re-export. (Compressing a second time is inevitable, but two is better than three, and if you start from a very-high-quality JPEG, is usually a non-issue.)