Does it make sense to change the fuel pressure in the common rail?
What can be achieved by changing?

Yes.
You achieve better power / torque, consumption and so on, it goes one with another...
You have to increase it if you want to inject more fuel in some normal duration before exhaust valves open...
But when you change rail pressure, you have to change several more parameters that you have to calculate.

thank you.
If I reduce the pressure in the rail at idle, will the engine run smoother?(at idle)

if you touch pressure also soi changes...so do not change pressure at idle...

Increasing rail pressure stresses the pump more so there is a safe limit. Reducing rail pressure does not make the engine smoother, just less NOx, and less power needed from the pump (small consumption hit), also increases smoke.

+1 Nice answer.

Thank You I understand!