Law enforcement and the prosecution present one version of facts. It's up to the defense to find problems with that version, not only by careful review of evidence, but also by independently gathering evidence of its own.
Investigators are independent professionals who gather information and evidence about cases. The prosecution is stacked with “investigators,” from police officers to investigators specifically employed by the prosecution. Most defendants, on the other hand, are lucky if they can afford even one investigator.
In addition to using court discovery procedures to obtain evidence from the prosecution, defense attorneys have a duty to investigate their clients’ cases. Effective lawyers will gather evidence of their own in preparation for trial—and even to see whether the client has a reasonably shot at winning at trial.
Interviewing prosecution witnesses can be a critical component of preparing a defense in a criminal case. But there are other forms of investigation that might also provide or reveal essential information.