Two of the most compelling reasons are cross-browser support and a better cross-device experience. For example, viewing/editing Keychain password on an iPhone requires burrowing into the Settings app, whereas 1Password has an excellent app and extension.
Works great until it doesn't (multiple user profiles in your browser, HTTP auth, non-browser based stuff like VPNs).
Wouldn't multiple user profiles have their own extensions? If so, then just install the extension on that profile? IIRC 1Password was working on something related to that, so perhaps that has changed recently.
HTTP auth not working is a bit annoying, but it's not a massive deal when you can CMD+ALT+\ and copy-paste it. Same deal with non-browser based stuff.