High-fiber foods are actually limited and recommended to be avoided oftentimes as these may aggravate symtoms/sides like diarrhea or sore mouth.  Dietary modifications like such are made in able to help the body strengthen and help to improve the bodies response to cancer and/or its treatments.  For someone with cancer, it is quite a challenge usually to get the proper nutrients, unlike the usual case of a healthy person.  Thus, changes like this are made in order to help the patient remain as healthy as possible, especially during treatments.

This is just one example of a very specific diet modification that is commonly made.  Numerous different changes are sometimes needed for people with cancer as side effects can obviously vary largely from person to person.