An argument gets out of control and the police are called.
Domestic violence is a real epidemic. Typically a fight, usually fueled by drugs or alcohol results in call to the police over the incident. Police interview witnesses and determine if probable cause to arrest one of the involved individuals. As the tempers settle with the dust a wife, husband, boyfriend, girlfriend or paramour realizes the consequences of what has happened and may have made false statements and now wants the charges dropped.
The Aftermath and a Zero-tolerance jurisdiction.
If the complaint lied and wants to undue the damage, it may be too late. If the complainant lied or made false statements, he or she cannot take the stand or they will be subject to a charge of perjury. So the rules of evidence provide that a husband or wife need not provide testimony. However, they will often not be needed as arresting officers will be able to testify as to what they witnessed and what was said.