Generally, the police investigation regarding a crime is concluded or close to being concluded at the time of arrest.  Usually, the police focus their investigation on gathering evidence that will help the prosecutor prove the case against the accused.  Also, the police are limited in resources.   So, the police do not always investigate all critical areas of a case.  Furthermore, not all police officers are ideal law enforcement officers.  In many cases, the criminal defense attorney's investigation into the case is critical.

Prosecutors never want to prosecute someone who is innocent of the crime.  But, prosecutors must rely on the police investigation unless it is shown to be flawed.  Since prosecuting offices are political offices, once a case gets filed as a criminal case in the clerk's office (and becomes public record), it is usually more difficult to get the prosecutor to overturn the original filing decision.

Many prosecuting offices have separate prosecutors who make the filing decision.   Generally, the time period between the arrest and the prosecutor's filing decision occurs within the first 21 days.  This time period is critical when trying to show the prosecutor that the police investigation is flawed.  Typically, prosecutors will review all information provided to the prosecutor's office.  The criminal defense attorney can communicate the importance of the information provided to the prosecutor.  Additional information could effect the prosecutor's position regarding the prosecution of the case.