How do you want to evaluate timeseries (e.g. 10,20,90,50,20) against your alert condition (e.g. >80) - should alert will be failing in this case or not?
No matter what you say, someone will always have a different opinion. That reduce will function will reduce timeseries to single value (e.g. last/min/max/avg/… of timeseries), so then it will be clear, which metric value will be used against alert condition.
I recommend to ask Grot about this topic if you need to know more details.