Why Do I Need to Periodically Set the Zero Offset?

Setting the Zero Offset is essentially the same function as taring a scale. Power meters are a lot like scales, in that they both measure force and work best when zeroed prior to every use. 

When you perform a zero offset, the power meter takes a torque measurement. Whatever this measurement is, the opposite is added to cancel it out. This is done to ensure that only the force applied by the rider is measured by the power meter.