Quote from pspr:
The 5th amendment is provided to prevent one from being forced to incriminate themselves. In order to plead it, one has to have something that would probably result in ones incrimination of a crime.
Therefore, most rightfully believe that when someone pleads their right not to incriminate themselves, they have something to be incriminated for.
Historically, the legal protection against self-incrimination was directly related to the question of torture for extracting information and confessions. The legal shift away from widespread use of torture and forced confession dates to turmoil of the late 16th and early 17th century in England. Anyone refusing to take the oath ex officio mero (confessions or swearing of innocence, usually before hearing any charges) was considered guilty. Suspected Puritans were pressed to take the oath and then reveal names of other Puritans. Coercion and torture were commonly used to compel "cooperation." Puritans, who were at the time fleeing to the New World, began a practice of refusing to cooperate with interrogations. In the most famous case John Lilburne refused to take the oath in 1637. His case and his call for "freeborn rights" were rallying points for reforms against forced oaths, forced self-incrimination, and other kinds of coercion. Oliver Cromwell's revolution overturned the practice and incorporated protections, in response to a popular group of English citizens known as the Levellers. The Levellers presented The Humble Petition of Many Thousands to Parliament in 1647 with 13 demands, third of which was the right against self-incrimination in criminal cases. These protections were brought to America by Puritans, and were later incorporated into the United States Constitution through the Bill of Rights.
Protection against self-incrimination is implicit in the Miranda rights statement, which protects the "right to remain silent." This amendment is also similar to Section 13 of the Canadian Charter of Rights and Freedoms. In other Commonwealth of Nations countries like Australia and New Zealand, the right to silence of the accused both during questioning and at trial is regarded as an important right inherited from common law, and is protected in the New Zealand Bill of Rights Act and in Australia through various federal and state acts and codes governing the criminal justice system.
The Supreme Court has held that "a witness may have a reasonable fear of prosecution and yet be innocent of any wrongdoing. The privilege serves to protect the innocent who otherwise might be ensnared by ambiguous circumstances."
http://en.wikipedia.org/wiki/Fifth_Amendment_to_the_United_States_Constitution
It's also useful for questions like "When did you stop beating your wife?"