Its not a by-pass capacitor though its value is very low. Its used to get the value of the capacitor very accurate as per my design. I have done experiments with my crossover designs that even a few fraction of difference in the values of capacitors / resistors / inductance causes a huge difference in filter phase , phase linearity and phase distortion. These caps / resist are used in series / parallel combination to prevent exactly that.
I have also noted that capacitors are always above or below the state values at least by 10% and hence needs to be measured and corrected before use. You will require to do this only if you have overall taken care of your design well else this will anyway not matter.