Use Case:

The customer wants an extra field "description of intended use" to be shown on the developer portal, on the page where a developer can subscribe to a service (Services > New template in the CMS).
While the field can be added to the corresponding template manually, the provided information can't be stored anywhere, as currently the Field Definitions are only available for Accounts, Users and Applications.

If custom fields could be added for Service Subscriptions as well, this field could be made required, and based on the answer the API provider can decide whether to approve or reject the subscription.