A person who represents themselves in a court of law is said to be acting pro se, a Latin term meaning "for oneself."
Our daughter is an attorney and spends a great deal of time preparing before her cases go to court. If one of her clients tried to walk right up to the judge and speak for themselves the judge would immediately have something to say about it and I don't think it would be positive.
In most legal proceedings, one does not just approach the bench unless asked to do so, or instructed to do so by the judge. Each client must have a mediator on their behalf, an advocate between themselves and the judge. Likewise, we cannot ourselves go to God to represent ourselves in regard to our sin. We must have an advocate, a mediator.
"My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous:" (I John 2:1.)
"For there is one God, and one mediator between God and men, the man Christ Jesus; "(I Timothy 2:5.)
The Man, Christ Jesus, is our advocate. He "ever liveth to make intercession for us."
Not only do we have an advocate, but He has also paid the entire penalty of our sin. And He is the beloved Son of the judge.