Every built-in browser password manager I've seen seems to be just a password-protected list of (URL, name, password) tuples. They're the most bare-bones possible implementation.

There's no (non-cloud) syncing, or easy backups. There's no file attachments, or custom fields, or non-website-based accounts. There's no automatic checking for duplicate passwords, or old passwords, or alerts for websites known to have been compromised.

I don't use a web-based password manager like LastPass because I don't really trust it (for either security or reliability), but local password managers are great for storing all kinds of private data. How would I store my driver's license, my storage unit lockbox combo, or my marrow donor registry ID in a web browser?

Until not so long ago, the browsers' password storages provided absolutely zero security. They are better now, but still password managers offer some advantages. For me the most important is the ability to use multiple browsers. But there's other stuff: random password generator, ability to store custom key/value pairs, 2FA, etc.

Having a password manager on mobile is needed, so I need multi-device support. I was really happy with the password manager support the last iOS included. Password entry and persistence are streamlined.

I use multiple browsers, and store credentials for things other than websites.