What is the effect of the optimal calibration pressure of the Italian coffee grinder on the espresso machine

It is enough to read somewhere and say 15 bar, and any pressure higher than this pressure makes no difference. The standard pressure of household machines is 15 bar, and that of professional machines is 9 bar! If so, why are there more stressful machines (16, 19, 20, 21 bar), and why do they cost more? If professional coffee is made from 9 bars, why do household machines use 15 bars to make coffee?

Make standard espresso!

9 bar is "standard", not 15. This figure is obtained after many attempts / errors and other pressures and is accepted as a standard by Italian and American associations and guilds. Some people even build equipment to check that their machines reach the accuracy of the pressure.

Professional (also known as commercial) machines can exert more pressure, but will automatically control without exerting more pressure. The pump they use is called a rotary pump, and because of its structure, the velocity has nothing to do with the pressure (the pressure will remain constant).

Household machines use cheap vibrating pumps. In them, the pressure is indirectly proportional to the flow rate. In order to make the pressure more constant and independent of the flow rate, an overpressure valve can be added. But they need a pump pressure greater than 9 bar to ensure that this 9 bar pressure will enter the filter basket in the portafilter.

One of the hallmarks of high-quality coffee is Klima. If the coffee is good, there will be a layer of Krima on top of it. The marketer knew that you would think that if my espresso had Krima, it would taste good (not necessarily true). As a result, they added a pressurized filter that can produce oil even with old coffee ground a month ago. As the name implies, pressurized filters require more pressure than 9 bar standard filters.