You can add secure notes to your iCloud Keychain account and login entries if you’d like to. The notes can serve any purpose, but they can be particularly handy if you want to reference something specific to that login, whether a log of prior passwords, or the purpose of the account, or whatever else.
For example, lets say you have a specific login to a credit card saved in iCloud Keychain, and you want to attach a note to that keychain entry stating it is a business account only, that’s easy to do. Or maybe you have a Netflix login saved, and you want to add a note reminding you it’s your favorite aunts account and not yours, that’s easy too.