I always assumed that it was to completely kill it and that resistance was a side effect of not killing it...
Stepping down off a high dosage in halves is the exact same strategy used to ease people out of addiction.
As a kid, I thought anti-biotics were this far-out medicine, so strong and hazardous that coming down off them was something you had to do carefully.
I got strep throat a few times as a kid. I don't have perfect recall about the details of what I had to take and when or why, though. But I do remember having to vary dosage.