Enabling two-factor authentication (2FA) for Webmail and Mobile Web is a prerequisite for generating app passwords for mail clients because it adds an additional layer of security to your email account. This ensures that even if your password is compromised, an attacker would still need access to your second factor (like a code from your authenticator app) to gain access to your account. Once 2FA is enabled, you can generate app passwords for mail clients that don't support 2FA.