The ViewingKey feature ensures that users can have visibility of a wallet but cannot spend the amount in it. A feature that could have multiple uses, as an example an organisation that is fundraising into a wallet and wants 5 different people to have oversight of the balance. It can be used as a form of accounting system too.
Using the ViewingKey for a z-address, a user can verify if that z-address has payments sent to it and to monitor the balance, but they wont be able to spend it.
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