They should inject insulin because their pancreas can’t make pancreas by themselves.
The pancreas admits body to use sugar from carbohydrates in the food which you eat for energy or store glucose for using next time. Insulin helps to control your blood glucose level from getting too high hyperglycemia or too low hyperglycemia.
If you get more sugar in your body than you need it, insulin helps store that in your organs such as liver and releases.
As a result, insulin helps balance about blood glucose levels and keeps them in a usual range.
If pancreas does not make sufficient insulin or your organs are refuse to the effects of insulin, you may develop