Sometimes it's used to uncover crimes, but very often it's used to invent crimes that never actually happened, or used to deceive a courtroom when they don't actually have evidence.
His neighbors yelled something to us out of their window. We went to see what they wanted, but they didn't say anything when we got near.
We went back to the porch and waited for maybe 10-15 minutes, but decided to leave as we felt really weird about those neighbors. As we started walking, a police officer pulled up screaming at us, telling us we were under arrest for throwing rocks at the windows of those neighbors. The police officer took us to the police station, put handcuffs on us, as well as shackled our feet to the floor, where we remained for like 2 hours. He made us write a statement, and then told our parents to come get us.
It was truly the most confusing thing I ever experienced. Angers me a bit today thinking about it.
Fabricating evidence in an interrogation to try to get a confession out of someone versus "I'm not a cop" when infiltrating a drug ring, or lying to hide important information
Lying about evidence specifically is illegal in a lot of countries (not the US)