To optimize your campaign means to make changes that will result in better results based upon tests and historical data. Costs don’t matter if people don’t convert; Lower cost per click/conversion doesn’t mean a campaign is optimized. For example, just because the cost per conversion decreased does not mean that it’s the right conversion. A lower cost conversion might be an Invidia who responded positively to the initial messaging but does not continue a free trial or cancels their subscription after one month for example.