Why do people have to undergo a hemi-colectomy? What are the main medical reasons for this operation?

Usually colon cancer, sometimes severe diverticulosis or a perforation, but the latter doesn't usually require that half the colon be removed.

It's fairly serious, but most patients do well if it is done electively.

because for some reason the bowel is better out than in. my daughter has had 2 of these operations she has crohns and the bowel had become so diseased it was useless. 2nd time was mainly scar tissue and adhesions from the first operation that required it.