Thr reason to use a password manager should be because passwords now need to be unique per login. Domain binding is a close second.

Unfortunately, as bad as phishing is, service providers have leaked more plain text passwords than a phisherman could ever catch.

Well yeah, that too. But I was doing that manually before anyways, didn't really change when I started using a password manager, except the passwords of course got a lot stronger since there is no need to remember anything.

But the domain binding just isn't possible without technical means, hence I see that as my own top reason, I suppose :)