Witness protection serves the purpose of protecting witnesses who testify in dangerous criminal cases.
The United States Federal Witness Protection Program is also known as the Witness Security Program (WITSEC). WITSEC was established as part of the Organized Crime Control Act, created to help encourage witnesses to testify against organized crime perpetrators.
Witnesses who testify against organized crime perpetrators, or other dangerous criminals, can be in immediate danger of retaliation after their testimony. The witness protection program is an effort to protect such witnesses.
As part of the program, witnesses are often relocated and even given a new identity. This means that they can get a new driver's license, social security number, name, job history, etc. The witness protection program is carried out by the U.S. Marshals Service.