Multiple gear ratio sets are desirable for the same reason as with any power unit. The power output and performance of an electric motor will be limited by the maximum rev's it can run at and the amount of electrical energy that it can draw from the power supply. A gearbox enables use of the rev' range and power band through each step change. Optimal horsepower and torque is therefore available for acceleration up to the eventual top speed. If only one gear is used either acceleration, top speed or both is unacceptably compromised. In addition, just like a combustion engine, an electric motor doesn't have to work so hard if the rev's can be kept high since the inertia of the unit's flywheel mitigates against power loss as the vehicle gains speed.
In other words a car with only one ratio may either be geared to make a high top speed with poor acceleration or a low top speed with massive acceleration. The ideal ratio might fall somewhere in the middle. Therefore a gearbox extends the speeds attainable whilst maximising acceleration. It also allows flexibility to have the right gears for different types of corner and track characteristics.