Think of your boiler as a car engine. If you want to drive at a steady speed of, say, 60 kilometres per hour, with an on/off system, you would have to accelerate fully and then stop accelerating entirely to try to maintain the speed. The result would not only be a pretty jerky ride, but also a waste of fuel and money.
In contrast, with a digital system, you could gently accelerate until arriving at the desired speed of 60 kilometres per hour. You could then slowly adjust your acceleration allowing for a smoother ride and maintain a more constant speed. You end up saving fuel and some money!
It is important to note that the digital setup will only work with a combi boiler. tado° can only control system boilers with a relay connection.