Requirements and capabilities should provide specific interface definitions. So that the designer of a app can depend on a contract set of public properties that are to be provided and a contracted set method/interfaces that are provided by the requirement.

For example:
If I have a dependency on a DB Server, and need to run SQL on that server, at design time, I would like to know the methods supported by the DBServer requirement so that I can leverage that in my component scripting.
On the implementation side providing the component, the script/method to execute the script may be different for each DBServer type. IE commands to run SQL on MySQL are not the same commands that would be used for Oracle. However, the SQL may the same in some cases.

Similar methods/use cases exist for J2EE server deploy methods etc.

By providing a interface on the Requirement/Capability we can leverage the workflows specific to the capability in a generic way. And the designer would be able to identify the contracted calls that are made available.

https://www.oasis-open.org/apps/org/workgroup/tosca/document.php?document_id=49635