Power equals Voltage by Current:

The higher the voltage, lower current is necessary to get the same amount of power (P=VI). 

The voltage should be managed by compromising safety and current rating of component.

This is a parameter which is set based on electrical characteristics of the motor/controller and wiring. 

Voltage has no mechanical effect on the ebike performance. 

Having a higher motor or battery Voltage does not translate a higher performance or higher range by itself. 

Voltage in Li-Ion battery is a matter of number of cells in series.

 

Battery Voltage will not be constant and varies based on SOC (state of charge) a complete charged battery voltage is about 42V when it is nominally 36V and battery voltage is about 30V when the battery is considered complete depleted.