When you mail a letter to your friend, you hope she’ll be the only person who reads it. But a lot could happen to that letter on its way from you to her, and prying eyes might try to take a look. That’s why we send important messages in sealed envelopes, rather than on postcards.
Email works in a similar way. Emails that are encrypted as they’re routed from sender to receiver are like sealed envelopes, and less vulnerable to snooping—whether by bad actors or through government surveillance—than postcards.
Source: Official Google Blog: Transparency Report: Protecting emails as they travel across the web