there are some cases in which a low-fiber diet may be necessary. A diet lower in fiber might be tried before having a colonoscopy, for diverticulitis, during a flare-up of inflammatory bowel disease ...

[15] All subjects were instructed to consume a low-fiber diet for 2 days prior to colonoscopy and were given the option of consuming a PEG-based preparation using one of the following dosing ...

The low residue diet is not meant to help you lose weight. Instead, it's supposed to help heal your gut. The word "residue" here refers to the undigested food that makes up the bulk of stool.