The primary purpose of return on ad spend bidding in AdWords is to ensure that you do not miss out on any potential profits do to limited budgets. If the primary goal of your AdWords campaigns is to drive to Commerce sales and your budget is $5,000/mo spread across every campaign in the account, that is most likely too low to buy all of the available traffic. To get the most benefit from AdWords, you need to have unlimited budgets for campaigns as long as they're producing a return on investment that is at or above your target. With Ecommerce tracking enabled, we can set a ROAS bid of, let's say 500%, which ensures that for every $10,000 you spend in AdWords, you return at least $50,000.
To see if you are missing out on potential Revenue, check out the competitive metrics column in your account and add search impression share. To get more details, add lost due to budget to review as well.
Read more about it here. Please let us know if you have any questions.