What is the point in hashing a value?
The purpose of a hash in this scenario to be able to uniquely identify an entity.   It's not strictly unique, only probabilistically unique.

Hashes are not reversible functions, so your client can't know the data that was encoded with it.  It could be guessed by brute force and maybe some know attacks to the hash assuming the type/format of data is known, but in principle is not reversible).

So process A (system or organization) A can work with the facts of the entity uniquely identified by the hash without knowing the personal identity value.  Process A can then pass the processed information back to process B, that knows the personal identity value and then can combine the information given to Process A and the information it already has and carry out another processing.

The advantage in this case from a security perspective is that you have less security issues as process A (system or organization) will never has access to that personal identity.  Even if A is hacked, the personal data is safe*.