It is important to properly shift gears on your ebike to get the most out of your ride and take care of your ebike motor and controller.
Once you have the equivalent of a horsepower motor added to your bike, it's hard to not just want to leave the gears in the highest gear and gun it from a dead stop in throttle only (recreating your favorite scenes from Fast and the Furious on your new ebike)! However, believe it or not, you should be riding your new motor-powered bike the same way your rode your bike when it did not have a motor installed. As you would never start from a dead stop in the highest gear (for top speed) due to the fact that you would exhaust yourself prematurely, not to mention that your legs would be jello in no time, the same is true after a motor is installed.
You can think of an ebike a lot like a car or motorcycle. The whole reason for having gears is to use the engine's power in order to efficiently match driving conditions from hill-climbing, acceleration from a standstill, or even trying to reach a top speed. When climbing up a steep hill or starting from a dead stop you should be in a low (larger sized) gear just as you would with a regular bike and shift into higher (smaller size) gears as you gain speed and require more mechanical advantage. This standard process of shifting down as you climb hills or come to a stop and shifting up as you want to increase speed is one of the foundational skills and key components of riding a bike (yes, even an ebike).
If you do not follow this common practice of shifting in accordance with your riding conditions and decide that:
"I want to ride this bike in the fastest gear always and never touch the shifter and don't care if this puts excess strain on the motor"
Then this is fine, however it will result in premature failure of your motor and/or controller. Just as you can blow a car engine by not shifting, the same is true with ebike motors and improperly shifting or not shifting at all can result in costly repairs.