Going on metformin as a pre-diabetic is about buying time and delaying the onset of diabetes. Studies have shown that starting metformin as a prediabetic can delay the onset of diabetes for up to ten years and will also help to delay any potential complications of T2 diabetes by a similar length of time. The earlier it's started the more time you buy.
I took 1500 mg of metformin each day for six years as a pre-diabetic and by doing so I seem to have slowed the progression of T2 by about five years. My HbA1C is better than I would expect compared to T2 family members at a similar age, and I can more easily keep it at a healthy level, so I believe it works. Considering that the life expectancy of a T2 in my family is 60, I will do anything I can to buy time.
If you are on regular metformin -- doesn't have SR or MR after the name on the box -- then ask for slow release metformin. The slow release version is much easier on your digestive system and most people can take it with very few of the side effects of the regular version. And stick with it, you may not think it's necessary to take metformin as a pre-diabetic, but five years from now you'll be happy that you did it.