People with diabetes often receive insulin injections. what effect does insulin have on the body?

There's two possible answers. 

People with type I diabetes, do not make any insulin in their bodies, so they have to received insulin injections in order to bring their insulin levels to the optimal level, which brings glucose levels down.

People with type II diabetes are making insulin in their bodies, but their bodies do not respond to it or respond well, that's why they need insulin injections so their bodies can use glucose for energy (insulin makes glucose blood levels down)