At this time of year I often walk into the flat where the boiler is, and it is not running, but red light is on. With me since the boiler does not modulate, I could time how long run light is on, but it would not really help, if it's cold, I want heating on.
People with modulating boilers could not do even that, only way to measure energy used is to measure how much oil or gas is used, latter easy enough, but dipping oil tank not enough change day by day to measure.
I can see the point with gas, the boiler can extract the latent heat, but often it does not due to the way it is switched on/off. With OpenTherm the thermostat could work it out, as it tells the boiler output required, however not looked into if the display changes when using OpenTherm?
But at least with my oil boiler simply can't see the point of the display, what do it tell me?