In any criminal proceeding, the prosecution must prove the case to the court, beyond a reasonable doubt.
This means, where a driver is charged with DUI, the judge must be 100% sure that the driver committed the offence.
Where a DUI lawyer can show a reasonable doubt that the DUI happened, or where the judge is not absolutely sure the driver committed the offence, the judge must dismiss the charge.
The job of a DUI lawyer is to disprove the charge, discredit the evidence of the prosecution and to create a reasonable doubt in the mind of the court.