Rationale
Ensuring a valid identity on an email has become a vital step in stopping spam (as email can be filtered based on such an identity), forgery, fraud, and even more serious crimes. The Simple Mail Transfer Protocol (SMTP) is continuously evolving, but when it was designed, in the early 1980s, it was the purview of academia and government agencies, and as such, there was no cause to consider security. It provided for no formal verification of sender.
Signing emails is a good first step towards identifying the origin of the message, but it does not establish whether that identity has a good reputation or whether it should be trusted.
This article explains how email identities are forged and the steps that are being taken now to prevent it.
Read more about this topic: Email Authentication