"You have the right to remain silent. Anything you say or do can and will be used against you in a court of law. You have the right to an attorney. If you cannot afford an attorney, one will be appointed to you. Do you understand these rights as they have been read to you?" This is to protect the individual against infringements of Fifth Amendment rights.
Anyway, here's the background of this warning. Ernesto Miranda was arrested for a rape of a mentally-handicapped 18-year-old girl that had occurred ten days earlier. Apparently, the police did not inform him that he could remain silent. As a result of the interrogation, Ernesto Miranda signed a confession; and this confession was used in his conviction. However, his court-appointed attorney appealed this conviction on the grounds that Miranda did not understand he had that right.
The Supreme Court, in the Miranda v. Arizona decision, overturned that conviction by a 5-4 decision. Because of it, police making arrests have fallen into the practice of using printed Miranda cards that spell out these rights so that any subsequent interrogation and possible confession can be based on a waiver of these rights.
Now here's the sequel: The diligent prosecutors were able to come up with eyewitnesses to the rape in question. Miranda was subsequently convicted at a retrial, and sentenced to 20 to 30 years in prison.
However, Miranda was paroled in 1972, after spending a few years in prison. He had a sideline business selling autographed Miranda warning cards at $1.50 each. Apparently, he was soon charged with small offenses and sent back to prison for parole violations.
His end came in 1976, when he was stabbed in a bar fight. The prime suspect was read his Miranda rights, kept silent, and he was released because of a lack of other evidence. The suspect knife-wielder did not waive his Miranda rights after they were read, and a possibility of conviction might have been lost!