SSL is only a means to pass data without third-party able to intercept them.
It doesn't improve security by itself. it only allows to communicate safely, at the cost of raising, slightly, server load.
SSL certificate has one more value: if issued by a well-known CA, the certificate may serve as a proof of good standing. In other words, it allows to create trust in the party using the certificate and secure data exchange it allows.
This is why self-signed SSL certificates aren't popular when the question is to trust the site or not.