Criminal case usually based on evidence; and there have two types of evidence, i) documental evidence and ii) circumstantial evidence.

When the case based on documental evidence, in that case role of the Advocate is very narrow, documents itself talks a lot, and nobody can go beyond the documents, but if there have circumstantial evidence then the Advocate play a vital role, because advocate can make out his case by using his clever mind, and changed the entire story only using his skill.