The likely answer is the Energy efficiency . The intensity of a sensory variable (such as light) could be encoded by the firing rates of neurons in the brain in two different ways:

• Set the resting firing rate of the cells at about the middle of the intensity range so that the intensities higher (or lower) than the middle range would be represented by increased (or decreased) firing rates.
• Use two sets of cells both with relatively low resting firing rate. These two sets of neurons encode respectively high and low intensities by responding to them with increased firing rate.

The energy consumption of a firing neuron increases as the rate increases, it is obvious tha tthe second strategy above is more energy efficient.