You've gained the convenience of not round-tripping through your email client every time you want to login/authenticate with the service!
The same is true of passwords, of course; if there's an email-based password-reset mechanism, then your email is your real proof of identity; the password is a shortcut.
Passkeys are a shortcut too; they just pick a different set of tradeoffs (less portable, less user-visible, more secure, harder to get phished, less stuff to remember). IMHO, these tradeoffs turn out to better-match the threat model of the modern web.
The same is true of passwords, of course; if there's an email-based password-reset mechanism, then your email is your real proof of identity; the password is a shortcut.
Passkeys are a shortcut too; they just pick a different set of tradeoffs (less portable, less user-visible, more secure, harder to get phished, less stuff to remember). IMHO, these tradeoffs turn out to better-match the threat model of the modern web.