The purpose of a suppression hearing is to allow the judge to determine whether a given piece of evidence or testimony will be ruled admissible in court. This usually happens if a lawyer has cause to believe that police acted improperly, violated their client's rights, or otherwise have a weak case, according to the website of the State of New York court system.
As described by the New York State Unified Court System, a defendant may ask for different types of suppression hearings depending on the specific nature of the evidence in question. A Mapp hearing, for example, is for considering whether or not evidence was seized in accordance with the law. A Huntley hearing focuses on whether the police acted properly in taking the defendant's statement or confession. Still another type of suppression hearing, a Dunaway hearing, determines whether the police acted properly while arresting the defendant. Regardless of the specific type of hearing, the judge decides whether or not the evidence at stake may be introduced by the prosecution. If key evidence is suppressed and the prosecution declines to file an appeal of the judge's decision to suppress, it is not uncommon for the charges to be dropped.