Which aspect of a certificate makes it a reliable and useful mechanism for proving the identity?

The aspect that a certificate can be a reliable and useful mechanism to prove one's identity is: the person's information and specimen like signature.  

Certificate or identity cards contains important and unique information about a person like social number etc. This is a proof that the person you are making transaction or activity has that identity. No other person would bear the same social security number unless one of them is a fake. Another unique aspect of certificates that can be useful in proving one's identity is his signature. Each of us has styles and ways on doing things like how we sign and write our names.