Not guilty buys some extra time to negotiate a plea deal.
It also gives the defense time to do their own investigating into the conduct of the cops, the legality of any searches and seizures

, and to look for mitigating circumstances that might excuse the criminal acts of the accused.
In the US, there's the truth, and there's the law. The truth might be you're guilty as sin, but the law still has to prove you're guilty in a court of law either before a jury of 12 or a judge (bench trial). Short of a written confession, that's not often a slam dunk. People caught red-handed on clear video have gotten off due to things like the video being inadmissible, it might show signs of tampering, etc.
in a world getting flooded by AI, the line between truth and pure fiction is beginning to fade away.