This metric is captured by Dynatrace in the Microsecond granularity by default, and when reported that can be mathematically grouped and added to show minute/second/millisecond times. As calculated service metrics cannot be adjusted after creation, this ensures the most detailed granularity within this metric so it can be used.
Because of this, this is a metric which can be used for Custom Alerting, and by changing the granularity it just tells the back end to group microseconds into mili/second/minute timings you are requesting.
In short, it prevents a custom metric from creating per minute intervals and trying to make Custom Alerts in second intervals.
Hope this helps