The URLs, tokens and headers a workflow's actions need to reach outside services.
Why here rather than inside the workflow
A token typed straight into a node's config sits in the workflow definition as plain text — visible to anyone who can view the workflow, and preserved in the graph snapshot taken when a run starts.
Stored here, the workflow points only at the record itself, and the real value is read at the moment it runs.
The value is never shown again
The data is stored encrypted, and this page shows only the name, the type and when it was created. There is no way to read the value back.
If a secret is lost, a new one has to be issued by the service it came from and replaced here — it cannot be recovered from this page.
Changing a value does not break workflows
The reference is to the record, not to its contents, so replacing the value is enough; no workflow needs editing. The next run picks the new value up by itself.
Renaming is safe too: the name is a label for your benefit and breaks no reference.
Deleting the record, however, leaves every workflow pointing at it without a secret.
Site scope
A credential attached to a site is available only to that site. One attached to no site is global and visible everywhere.