PKI certificates weren't even intended for SSL, it predates even that.
X.509 was published in November 25, 1988 ; version 3 added support for "the web" as it was known at the time. One obvious use was for X.400 e-mail systems in the 1980s. Novell Netware adopted x.509.
It was originally intended to use with X.511 "Directory Access Protocol", which LDAP was based on. You can still find X.500 heritage in Microsft Exchange and Active Directory, although it's getting less over time and e.g. EntraID only has some affordances for backward compatibility.
Are we really at an age where people don’t remember that SSL was intended for many protocols, including MAIL?!
Do you think email works on web technology because you use a web-client to access your mailbox?
Jesus christ, formal education needs to come quickly to our industry.