The main benefit is you will never put your passkey on a phishing site. Password managers provide some protections against it because if they do not work automatically on a website you know something is fishy, but sadly many websites have botched their password input so even with a password manager you may still need to manually copy and paste (or even type, if pasting is disabled) the password.

The problem is whether or not the benefit outweighs the additional risks introduced — losing account access when you lose a device, furthering device lock down, difficulty transferring the passkey between devices, UX degradation due to bad implementation. In my opinion the answer is no and I am sticking with my passwords.