I asked our Alerting expert and seems like Orion.AlertActiveObjects is used in case of cumulative alert.
Cumulative alert is alert like this: alert me when 5 or more objects met the specific alert condition.
You can find more information about cumulative alerts here: Wait for multiple objects to meet the trigger condition
When such alert exists then active objects related to this alert are stored in table Orion.AlertActiveObjects.
So Orion.AlertActiveObjects is used only in these complex alerting scenarios and otherwise is empty.