Automatic machines have 4 individual programable buttons. After the coffee is placed in the machine, the button corrisponding to the desired level is pushed. This engages the brewing cycle that will measure the coffee brewed and will shut off at the disired level. Here are some reasons why. They also have different size boiler capacities.
Larger boilers have more steam capacity and much less down or recoupe time.
Today, steaming ability is a primary factor in a espresso machine purchase.