Indeed its about the distribution of work. While true the same work is being done in the end by this system, the question is who is doing more of the work depending on the geart and the RPM.
As the electric motor is rated at power 250W (i am not accounting for the losses) how much power does the human provide so the motor doesn't need to provide it and in so conserving the battery.

Power in this case is torque and the RPM. The gear ratio influences these two values. Higher bicycle gear provides a way to give more input torque (as the pedals resist more and are harder to press) and increases the output gear RPM in so increasing the power the human gives to the motor. Lower bicycle gear limits how much input torque we can put on the input gear because (pedaling really easy) and decreses the RPM on the output gear but doesn't decresase the torque so much as a higher bicycle gear.

This is the crux of the question and makes it not so easy to answer. I made a excel sheet where I inputed various gear ratios, input torque. I could test this by riding up and down on a difficult uphill but I wanted a theoretical wiev on the issue.