How could an informant affect an investigation?
Informants, or Criminal Informants as they are often called, most usually are people with past criminal offenses that then go on the payroll of law enforcement agencies in exchange for information they bring to the attention of police investigators that either solves cases or prevents future crime. Because these informants already are familiar with and are recognized in criminal circles, and often associate with other criminals, the information they provide is often very helpful in solving or prosecuting a case.
On the other hand, since these people are criminals who want money or a lighter sentence, you could make a good legal argument that their testimony or their information should either not be trusted or taken with a grain of salt, so to speak. Police often rely on these informants because they know that, if the CI gives them bad information, they won't hire them again, so they take the trouble to make sure the information they turn over is reliable much of the time.