when a user task is generated, there are the ID links that get created for things like candidate, assignee, etc.
But in more complex environments where there are third-party systems interacting with the REST API, the external systems often want to know more information about the user. Identity Links that provide more context about the related Identities of a user task. Could be the Departmental Owner (different than a "Task Owner"), the Backup Assignee, Quality Assurance Owner, etc. These are Groups and Assignees that could be used by the outside systems to enable permissions, feature toggling, etc.