On top of the body processing some of the alcohol in the time between the field test and the final test, the field test is usually not accurate enough to be accepted by the court for conviction. It's just one piece of the probable cause used to arrest a suspect and bring them to the police station or jail where the larger, more accurate machine can get a result that's usable at trial.

It's important the the test field test be fairly accurate right around the legal limit. At much higher numbers, it's fine for it to be pretty far off because the suspect is definitely getting arrested at that point.