Mutated cell proliferation
There are some diseases in which a minority population of normal cells can rescue the organism, even in the presence of a majority of mutant cells.  Sort of like a group vacation to Kazakhstan with one friend who can speak Kazakh.  Very different experience from a vacation where none of you speak Kazakh. 

Take for example hemophilia A.  Cells have a mutant gene for factor 8.  Without factor 8 blood does not clot properly and people have bleeding trouble.  You do not need much factor 8.  Just a little bit can prevent spontaneous bleeds.  People who are heterozygotes for hemophilia A (carriers) are fine at 50% normal.  So if you edited mutant liver cells such that they could produce factor 8, even if they were only 10% of the total cell population of the liver, having some factor 8 would sidestep the full hemophilia phenotype. 

Likewise metabolic mutants - some children are born unable to handle certain substances.  Phenylketonuria is the one you read about every time you drink a diet coke.  If a minority population of cells could be produced that were able to handle phenylalanine, those would rescue these kids even if most of the cells still could not.