Registering the non-canonical properties allow you to check/diagnose configuration errors. For example, if a canonical redirect should fail (or be incorrectly implemented) for whatever reason or SSL cert renewal failures and/or pages aren’t being indexed on the canonical property, you have no way of knowing (without doing a
site: search) whether those pages have been incorrectly indexed on a non-canonical property by mistake.
example.com www.example.com http://www.example.com http://example.com https://www.example.com https://example.com
That’s 4 properties (not 6), the first two are the same as the second two.