Why should I take metformin with food? Will it lessen the effect if I take it with only coffee and?

Metformin is hard on your stomach and the reason you need to eat Food is you need something on your stomach to prevent Nausea. Plus the food also helps your Metformin move through your stomach. Coffee is not sufficient if you are not eating anything.
In your post you listed "Bannana" and as far as eating a Bannana you have to remember that Banana's have alot of Carbohydrates so it might not be the best choice as it raises your blood sugar. When I take my Metformin In the evening I usually eat Peanut Butter as it has alot of Protein in it.
I had some issues with severe Naseau bouts awhile back and the first thing my Dr. told me to stop was the Metformin because it is hard on your digestion system.