You shouldn't expose it because people who see it will start to use it as their 'account number' which it is NOT. For example, for my bank account I know what my account number is. I've memorized it, I use it on the phone with customer service, I use it when filling out forms for other banks to do transfers, for legal documents, for my auto-pay service, etc, etc. I don't want it to change. The primary key (for my account) on the other hand, I don't know or ever see.
The system that stores it changes over the years from one system to another, through bank merges, system upgrades and replacements, etc, etc.
The primary keys may change through some of these transformation, so if its never been exposed, written down or remembered by any regular user that's a real good thing.

btw, this even happens internally when people built interfaces and programs that misuse and expose primary keys and make them part of such systems instead of them just doing one thing - uniquely identifying a database record internally. I actually learned the above through a 6 year stint supporting a data warehouse system in a hospital.

You shouldn't expose it because people who see it will start to use it as their 'account number' which it is NOT. For example, for my bank account I know what my account number is. I've memorized it, I use it on the phone with customer service, I use it when filling out forms for other banks to do transfers, for legal documents, for my auto-pay service, etc, etc. I don't want it to change. The primary key (for my account) on the other hand, I don't know or ever see.
The system that stores it changes over the years from one system to another, through bank merges, system upgrades and replacements, etc, etc.
The primary keys may change through some of these transformation, so if its never been exposed, written down or remembered by any regular user that's a real good thing.
Keys with no business meaning are often termed surrogate keys and are often (but not always) used as primary keys.

btw, this even happens internally when people built interfaces and programs that misuse and expose primary keys and make them part of such systems instead of them just doing one thing - uniquely identifying a database record internally. I actually learned the above through a 6 year stint supporting a data warehouse system in a hospital.