The higher the voltage, lower the current is necessary to get the same amount of power (P=VI). The voltage should be managed by keeping safety and current rating of component in mind.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). For a 36V rated battery  the fully charged voltage will be around 42V while the completely drained voltage will be around 30V.