The islets of Langerhans are clusters of cells present in the pancreas. One of the type of cells in these clusters are the "beta"cells. These cells produce insulin. When these cells are destroyed, the body cannot make any insulin and the person develops Type I diabetes or what was once called insulin dependent diabetes. These people cannot survive unless they inject themselves with insulin every day.
In India, people with Type I diabetes are a small minority amongst all the people with diabetes. People with Type I diabetes make up around 1-2% of all those with diabetes. But when one is talking of 25-30 million people with diabetes in India, the numbers are huge! The rest of the people with diabetes have what is called Type II diabetes. This was previously called non insulin dependent diabetes. This was unfortunate as it made people feel that they would never need insulin as a form of treatment. This is far from true. Although many people with Type II diabetes have adequate insulin and the problem seems to be that the body shows resistance to its action, a substantial number of those with Type II diabetes, especially in India, are what we now know to be people with Insulin Requiring diabetes. In simple terms, they have some capacity to secrete insulin, but this amount is not enough to meet the needs of the body.
When they are given oral tablets for their diabetes, they will show some control, but this control will never be ideal. The reason for this is that all the oral tablets which we have for diabetes are NOT insulin. In fact ORAL insulin does not exist as a form of treatment! What most of these tablets do is that they make the body a little more sensitive to the insulin which the body secretes. But there is a limit to how much this sensitivity can be increased. If the body does not secrete the minimum amount of the needed insulin, oral tablets are not going to be able to bring about the necessary level of glucose control, no matter how much they may be able to increase the sensitivity of the body to its insulin. Thus, people with insulin requiring diabetes also need insulin injections for good control. Even the other Type II people may need insulin as the beta cells fail after some years.
After all, you can whip a lame horse and make it walk a little faster, but it can never win the Derby!