What have countries gained by increasing/decreasing mains voltage by 10V?
It is to standardise everything on a single fixed voltage. But in practice, mains voltage isn't 230V. It is standardised at 230V +10% -6%, or 216V to 253V. Here, mains voltage measures 249V, which is on the high end of the scale - but we're on a small farm road with not many other customers. Away from home I have measured it at 233V, due to resistive losses from greater demand. Countries have not changed the actual voltage - that would require changing all the generators or all the transformers in the country (at a substantial cost), merely, the allowable range has changed.