Often, when you are diagnosed with diabetes your blood sugar has been very high. And this can totally exhaust your poor pooped out pancreas. It is just overwhelmed. The insulin can give your pancreas a reprieve while you get your diet, exercise and medication in order. And some of us are lucky, your pancreas can regenerate and as long as you are careful about things, you can actually stop using the insulin and do fine.
It used to be that doctors were very hesitant to start patients on insulin and many struggled for many months trying to get their blood sugars down. Your doctor was actually forward thinking. I know that you are worried about maintaining your blood sugar control. But if you can keep and A1c of 6% and stay healthy and not need insulin for years, why not? If you really do need that insulin, it is always there.
ps. I take 2500 mg of metformin, you could actually take more medication before needing to explore whether you need insulin.