It doesn't help a system-level adversary, but it does help prevent trivial takeovers by malicious actors attempting to get access to your email as a vector to c

It doesn't help a system-level adversary, but it does help prevent trivial takeovers by malicious actors attempting to get access to your email as a vector to compromise other services. To summarize, 2FA does not prevent email reading if the provider doesn't, however it does help prevent run of the mill takeovers, especially if you've reused a password somewhere. Security is all about defense in depth, it's worth keeping in mind that 2FA is an important step there, but by no means the only one. If you aren't encrypting+signing a message, you've already decided that security requirements of that particular message is minimal.