It points very strongly toward intoxication. There is just enough false positives that they are required by the state to also take a blood test.

It's a confirmation, and when you're about to sentence someone on the spot with what is often a life changing, multi year disqualification from driving, the courts require the police to be absolutely sure.

You're usually free to refuse the blood test, but the DMV is also free to revoke your license arbitarily.