I don't understand how simple DWI testing is like that in your country. 3 seconds of a certified calibrated breathalyzer is sufficient, this walking in a straight line and saying the alphabet backwards sounds like a joke.
The I stands for impaired. There a zillion other things that can impair your driving.
There are other forms of intoxication beyond alcohol. A device that measures your blood alcohol percentage does nothing for the driver who is half asleep from valium. A field sobriety test is more of an indicator of whether you are capable of operating a vehicle safely than of having a high alcohol intake recently. If you can't perform simple tasks, you probably shouldn't be operating a vehicle regardless of the cause.
The portable breathalyzer is inadmissable in court in my and most states. The Simon Says game is though (but it can be refused without penalty, hypothetically).
As others have said the intent is not to document sobriety but to have a subjective reason for an arrest which looks good in the scorecard.
Look for “if cops say I smell Alcohol, say these words” on YouTube, gives you tips on how to respond if asked about alcohol use or doing a sobriety test.