I use unique passwords for everything, and most of them I do not write down. I just let Keychain manage them. As I'm sure you know, "i hacked your email, give me $800 in bitcoin" emails are all the rage these days. I know they are fake, these passwords are not my computer passwords or email account passwords. They are from online accounts, and I'd like to know WHICH online accounts.
So it would be very handy if I could look at the password, search keychain, and use that to determine which website I have an account on, had their security breached.