One of the most important cases that influenced the rules of the evidentiary process was Miranda v. Arizona. It was from this case that we get the requirement to inform suspects of their rights in a specific way.
Miranda v. Arizona was a Supreme Court case in 1966. The case focused on the Bill of Rights, and specifically the Fifth Amendment. The Fifth Amendment guarantees Americans against the right of self-incrimination. Among other important rights, the amendment includes this important one.
No person shall be … compelled in any criminal case to be a witness against himself. (Wikipedia)
For this reason, the plaintiff Miranda claimed, the cops cannot require a suspect to answer questions. This resulted in the Miranda warning given to every suspect arrested on suspicion of a crime in the United States since. Suspects have the right to remain silent and consult an attorney, and they are reminded of that right the minute they are in police custody.
This is an important case because although most people know now that they do not have to answer police questions, being arrested is a frighteningly disorienting experience. Often police will coax confessions out of people before they consult a lawyer, and since this case these are inadmissible in criminal proceedings unless the Miranda warning has been given.