When a client wishes to employ a new attorney, many jurisdictions require the client to file a document explaining to the court that the substitution is taking place.
A client may fire and obtain new counsel at any time, but the substitution of attorney document helps the court and opposing counsel know who to send communications to.
A client’s right to fire an attorney and hire a new one, at will, is a time-honored recognition that non-lawyers are vulnerable to mistreatment by lawyers due to the complexities of the practice of law. Granting the right to fire attorneys whenever a party wants is a good way to protect a party against this vulnerability.
When a party retains new counsel, a substitution of attorney document puts the court on notice that the party will be represented by a new attorney. Practically speaking, there are times when the court and opposing counsel are prohibited from contacting a represented party directly, and confusion would result without a substitution of attorney.