What is the point of HIPAA de-identification + re-identification?

Section 164.514(c) of the HIPAA of Privacy rules discusses re-identification of PHI.

If a covered entity or business associate successfully undertook an effort to identify the subject of de-identified information it maintained, the health information now related to a specific individual would again be protected by the Privacy Rule, as it would meet the definition of PHI. Disclosure of a code or other means of record identification designed to enable coded or otherwise de-identified information to be re-identified is also considered a disclosure of PHI.


Under these rules, what is the possible purpose of de-identifying while retaining a method of re-identifying?

I could imagine someone wanting to store tightly secured HIPAA-protected information separately from less tightly secured non-HIPAA-protected information.

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But the except says that the linking key is itself considered PHI. So the de-identified information (including its key) is no less HIPAA-protected PHI than the the original data.

What is the possible case for de-identifying and re-identifying since both data sets are treated as PHI?