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Why does a not-guilty plea usually lead to a criminal trial in Anglo-American law?

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Mosaic portrait of Emperor Justinian I with a jeweled crown and halo, from the 6th-century Basilica of San Vitale in Ravenna, Italy. — Procedural law Justinian I, detail of a 6th-century mosaic at the Church of San Vitale, Ravenna, Italy.

In Anglo-American law, a criminal trial is held specifically because the defendant has contested the accusation by entering a plea of not guilty.1

The Anglo-American criminal process is characterized by an adversary procedure, where allegations made by the prosecution must be resisted by the defendant and determined by an impartial trier of fact, such as a judge or jury. Because the prosecution bears the burden of proof for all relevant facts, a defendant cannot be found guilty as long as proof has not been supplied or if reasonable doubts regarding guilt remain.23

If a defendant does not contest the charges—often through a plea bargain where they plead guilty—the court may bypass a trial, pronounce the defendant guilty, and proceed directly to determining the penalty. Therefore, the trial serves as the essential mechanism for the state to meet its burden of proof when the accused exercises their right to challenge the charges.123