I wonder if you have ever worked around acute care medicine. Almost all areas have some kind of an on call system. EMS and fire departments are famous for this as well. Scheduling for every conceivable situation is not cost effective, having on calls is.
For example: Ambulance company I worked at
4 Regular ambulances on 24/7
3 Regular ambulances on call
Regular ambulances:
If the 3rd unit was dispatched, the first on-call crew was paged. They had 30 min to get in. 4th unit dispatched, and 1st was more than 15 min from clear, 2nd on call was paged, and so on. It was rare to have more than one on call crew around but it did happen a few times a month.
One of the things about medicine is, when the major shit hits the fan, you could have every body the hospital owns in the trenches and still not see the bottom of the work pile for 12 hours. Forecasting for major freeway pileups, structure fires in large apartment buildings, airline crashes, natural disasters, is just not possible.
They can look at the averages, and having been trained in inventory and production management I understand the math, and it is still tough.
Lets say Fresno County CA dept of forestry fire crews respond to 200 wildland fires a year. How many crews does each fire need? Lets say 3. So now we have 600 crew units/year involved in fighting fires for an average of about 1.5 per day.
OK so we put on 2 crews
But wait we need 3 for a fire.
What if its a larger than average fire, we need 4 or maybe 6 crews. So do We always schedule 6 and let 4 of them play ping pong all day most of the time?
On call crews allow you to have 10x the resources available for the price of 1 full time crew. So when the “BIG ONE” comes, you can hit it hard and fast.