When you send an email without the encrypted settings set in Gmail, your email messages can be read in plain text during transit. If you use web-based email it is better from a security or privacy perspective to use HTTPS (if it is available). "If you sign in to Gmail via a non-secure Internet connection, like a public wireless or non-encrypted network, your Google account may be more vulnerable to hijacking. Non-secure networks make it easier for someone to impersonate you and gain full access to your Google account, including any sensitive data it may contain like bank statements or online log-in credentials. We recommend selecting the 'Always use https' option in Gmail any time your network may be non-secure. HTTPS, or Hypertext Transfer Protocol Secure, is a secure protocol that provides authenticated and encrypted communication.
To enable this feature in Gmail: