Just like how we would buy Coins, Instead we could buy Credits. It would benefit the Dev, and help us out in a pinch and running calls doesn't give us the amount we need at that time.
I generally run enough calls to get coins that I need. But let's say that I want to make another couple stations to cover an area and I can't find the time to earn a couple million credits then buying coins would help me out.
Or another option could be that a player can buy coins and use those coins to buy credits.