Name the gland that secretes insulin. Why are some patients of diabetes treated by giving injections of insulin?

Insulin is secreted by the beta cells if Islet of Langerhans located in the pancreas. 

Insulin helps to lower the blood glucose level in our body. 

When it is secreted in less amount, the body suffers from diabetes, as more and more glucose accumulates in the body. Therefore, in order to reduce the level of glucose in the blood of diabetes patients, they are provided with insulin injections.