The point of software like Lightroom and Aperture is not to convert RAW files. (In fact, there is some debate that the free manufacturer provided converts do a better job in some aspects.) The point is to catalog and manage the entire darkroom / post-shoot process. You can rate, organize, search, add keywords, change meta data, do basic touch up and manipulation, produce galleries and prints, handle exports, etc, even manage loading images in to Photoshop for more extensive, destructive edits, all from within Lightroom.

It happens to provide an option of doing RAW processing because RAW processing is a step in the process, but that isn't the point of the software. The point of the software is a quick, non-destructive workflow for the bulk management of a large number of images. You can't do that with Photoshop or GIMP.

Personally I use Lightroom for 3 main reasons. I can use it to apply approximate color corrections and exposure settings to all shots that I took under the same conditions at the same time. I can use it to rate my photos quickly and export according to those ratings. Finally, I can use it to easily find images out of some 50,000+ images I have on my computer due to the keyword and search functionality. None of that functionality is provided nearly as cleanly by Photoshop or GIMP.