So instead of a KeePass with a long random password for each site, I'll have a KeePass with key pairs for each site.
The added security being that if the site gets compromised, they can't log into the site using my account. But then again the site is already compromised at that point.
If you already use random passwords you get less security benefits, but you still get a lot of them.
The additional security happens if you consider MITM attacks. Assume that you want to login to https://mybank.com but somebody links you to http://mybank.com (bad dns) or https://mybank.co. With UAF or U2F the local authenticator will not sign anything (and thus the attacker can not login to your actual bank).
Now admittedly you browser plugin might warn you in some cases. However you might copy in the password by hand because you think its some kind of problem with the plug in.
The usability is of course improved as well. Their should be no need for a password manager at all. Go to the website, put your finger on the sensor and your done (or whatever other local authenticator you use).
The added security being that if the site gets compromised, they can't log into the site using my account. But then again the site is already compromised at that point.