Think of it like this: what you have with these Npubs (N being X, Y or Z), is your bitcoin wallet, with the only difference that it cannot be used to sign transactions, that is, transfer bitcoins. So why would you use something like that? Imagine that you are a person who tends to receive a lot in bitcoin, either because you sell a product or because you provide some kind of service. You could use a regular wallet on your phone, but if it gets stolen and the person manages to open the app, they still get access to your bitcoins. However, if you have a watch-only wallet (in which you use these Npubs) even if the person has access to your wallet, he will not be able to move any balance. He'll be able to look at your balance, know how much you have, what your addresses are, including all your UTXOs, but he won't be able to sign any transactions.