That’s a good question, since with it, one avoids the ’safety issue’ warning for using the same credentials for more than one site, when such a case is detected
It used to be like that in keychain previously and I had to check that functionality to be sure to respond correctly, as I was curious myself too.
It appears that when you have credentials for one site and want to use them for another site, you can do so. You’d select the credentials in the prompt on your iphone, by pressing the little key. Then you’ll get a prompt to add the url to that passkey. Automatically, it will be updated to use the same login for either site.
Here’s an example of the result, where i added two other amazon domains (.co.uk & .com) to the initial one (.se) in the passkey item.