Client cost metrics are useful if you need a report that you can share with your client, and want to use terminology that's more appropriate for their purposes. For example, with client cost metrics what Display & Video 360 refers to as "Revenue" is instead called "Cost" (since this amount is the cost charged to the end client).
Can I send this report to my client?
The name changes are useful for tailoring terminology to clients, but reports can disclose instances of Media fee and CPM fee. So depending on your configuration, these fields may include information (like Display & Video 360 fee or margin) that you don't want to provide.