For we, mere mortals, its chief merit may be that it's shorter.
What are colloquially known as SSL certificates should be referred to as X.509 certificates.
The term SSL certificate became common due to the adoption of the X.509 (one of the ITU X.500 Directory standards) certificate format by Netscape when it designed the original versions of the SSL (Secure Socket Layer) protocol, eons ago, when the world was still young, dinosuars still roamed, and the Internet was a friendly place.
Creating self-signed certificates is presented as a worked example of the use of the Open SSL package.
You can either buy an SSL (X.509) certificate or generate your own (a self-signed certificate) for testing or, depending on the application, even in a production environment.
It went through various iterations and is now at version 3 (dating from 1995) and used in a variety of clientserver applications.
Since the demise of Netscape the SSL specifications will not be updated further.
The term 'SSL certificate' has persisted, and will likely persist for the foreseable future, because given the choice of saying 'SSL certificate' or 'X.509 certificate' the former tends to roll off the tongue more comfortably.
Doubtless a linguistic expert could wax lyrical over the S sound versus the X sound.
Currently published RFCs are pointed to https:// XXXX which contains various information and links to the text (normative) reference and a PDF (non-normative) version.