What is the purpose of allowing comments inside email addresses?

Comments inside email addresses are supported mainly so that a deprecated form of email addresses - Specifically, "[email protected] (John Smith)" instead of "John Smith <[email protected]>" can be handled.

This old-style notation defined parenthesized content as arbitrary comments as has been pointed out here and referenced in RFC 822 as part of the general commenting convention:

http://www.freesoft.org/CIE/RFC/822/16.htm

.. So as you can see, the original email format has its roots from the early RFC 822 standard formed in 1982 to replace the ARPANET standard RFC 733 which also used parenthesis as comments. Eventually, brackets were standardized upon, however support for the original format has remained.

RE: https://en.wikipedia.org/wiki/Email#Message_format