Why?
If you’re in the US, you should know this because if you want to apply for a passport, you’ll be required to provide information about your parents such as birth dates and places. If you’re divorced, you will have to provide the same info along with marriage and divorce date, even if it was decades ago. So if you have access to that info, make sure you record it somewhere safe for Future use.
If you’re not in the US, you should know because this information can be difficult for people to get if they never knew one or both parents, or have a bad/non-relationship with them. Or if they had a contentious divorce or an abusive partner. Which is another reason why just leaving the country can be difficult for people who are already marginalized.


As an American/Swedish dual citizen…
I think I’ve only had to show mail to prove my address when first getting a driver license in a new state. So that’s a thing yes, but not very common.
Unlike in Sweden, in the US you don’t register/update your address with the authorities when you move. It’s not that the US doesn’t have a “working database” for that – it’s just not a thing at all, there’s no population register like in Sweden.
In Sweden you use your personnummer for identification, but you also have secure authentication methods like BankID that aren’t available in the US. Your personnummer is public information and you’ll provide it just about everywhere because there’s little risk to you.
In the US we use our social security numbers for both identification and authentication. Because they’re used for authentication, they’re considered secret and we’ll only share them when strictly required for necessary services (like government agencies and banks). This is obviously really poor security and they weren’t originally intended to be used for authentication, but it is what it is.
Swedish system is of course more efficient and more secure.
I have to show my water bill to dump my monthly free truckload at the landfill, but that’s about it. Sometimes utilities require it too.
Thank you for clarifying this!
The US badly needs a population database, however, I absolutely understand that with how the US government have abused and fooled their citizens in the past, there is a huge distrust of the government among Americans.
Yeah people won’t even fill out the census. Last time it was done most of the people I worked with said they wouldn’t fill it out.