We had a dog with inflammatory bowel disease that needed hydrolized protein food--essentially his IBD meant that he was allergic to protein. When you hydrolize protein, it breaks it into small pieces that the body doesn't recognize as protein.
However, the IBD gave Evan diarrhea...it sure didn't constipate him! Don't know how different it might be in cats, but I can't know of any other reason to feed hydrolized protein except for IBD
I hope your kitty feels better soon, Loki Love!