The US constitution says that police officers need a reason to search the body or property of a person, if the police officer suspects you of being drunk while driving before searching your body for alcohol they need some clue, not a proof but something subjective anyway, the field sobriety test is designed to provide that, its point is to confuse you and make you fail, when you eventually fail they can breath test you.
There are two main reasons. The first is that a roadside breath test only detects alcohol but a driver can be impaired by other substances.
The second is that the actual results of a roadside breathalyzer are generally inadmissible in a criminal trial outside of specific circumstances. Normally an officer can only testify that the PBT detected the presence of alcoholic beverage, not the actual number. I was a local cop at two different agencies, neither issued PBTs for this reason.
Case law in the US considers the roadside olympics to be more accurate than the roadside breath test.
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