>>214372821 (OP)
Usually the killer is involved from the very beginning of the case, having given the police their alibi even before Columbo steps on to the crime scene.
Lt. Columbo does his magic, gets the criminal to confess. And perhaps this confession won't hold up in court, but what about their alibi? Even if Columbo doesn't have bulletproof evidence of the killers guilt, he has certainly proven their alibi to be false.
So they've got a couple options in court. Attempt to re-use the old alibi and somehow convince the judge and jury that Columbo is simply wrong, attempt to create an entirely new alibi and get questioned on why your story has changed so much, confess to the murder, or simply shut up.
Proving someone lied about NOT committing a murder counts for a lot, even if you still can't definitively prove that they did commit the murder.