The easiest and cheapest way to power an ebike is with the use of throttle. The earliest ebikes were throttle only and these ebikes were running on heavy lead acid batteries. You can modulate the power exactly on how much you need, especially when starting from a dead stop. However, once you get moving you only need partial throttling and your hand gets tired and numb. Unsteady partial throttling consumes more energy.
I had the throttle only full suspension Rayos ebike and I can understand why pedal assist makes more sense. Pedal assist coordinates with your own leg effort. However, it's from the dead stop that throttle has the best utility since you can safely and forcefully accelerate while you are still in the process of raising your legs to the pedals. Torque sensors help a little in that situation but the exact metering of the power can vary from person to person and the amount of power provided is not as precise as using a throttle.
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Once the ebike is already running, PAS is better than using a throttle. I added a crude controller for my PAS that is linked to the throttle itself
and neutralized the return spring of my throttle so it can stay open as the power passes through the rudimentary dial type controller (PWM modulator).
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The simple answer as to why we need throttle is it is very cheap and very easy to operate and very useful especially when crossing a street. The same reasons are also used to ban it since many low quality and unsafe ebikes use throttle only.
For me the throttle is an innocent character that got dragged into a messy ebike regulation.