What is the purpose of the series of actions: amplification, band-pass, rectification, and low-pass?
The signals from the sensors were tiny, and needed to be amplified.  Otherwise they would be lost in the noise in the rest of the system.
When "interesting" things happen, neurons fire rapidly.  Filter out any signal that isn't firing rapidly.
We want an amplitide, but an AC signal has an average amplitude of zero.  So make everything positive.
Filter out the rapid firing (> 400Hz), smoothing it out to give the overall "envelope", showing the strength of the signal.