A torque sensor can increase the range of you e-bike. And they will keep the law enforcement from classifying your vehicle as a moped. But they are not cheap.
Torque or strain sensors, so the bike cannot be driven with out pedaling, are not absolutely necessary but every thing helps when it comes to building a legal vehicle.
But you don't really to need to sense the pressure on the pedals, you can use a Cadence sensor, it is the same as the speed sensor; when the crank arm passes a magnet it sends a pulse to the controller.
However torque sensors apply power more smoothly than a cadence sensor. On a quality torque sensor, you can forget the bike is electric, the bike just becomes easier to pedal. The harder you push on the pedals, the faster you go like on a regular bike, but with the electric power you just go a lot faster.
Electric bikes are generally super inefficient when accelerating up to speed. A torque sensor forces you to help the ebike when it needs it the most, when it is accelerating and when climbing
In the state of Washington a speed sensor connected to the controller to calculate wheel speed, is absolutely needed if your gear ratio, in conjunction with the motor's maximum RPM, is enough to move the bike faster than 20mph on flat land. That is one reason why washington state e-bike laws must change!
PAS (Pedal Assist Sensor) installation problems
washington state e-bike laws must change