Why do we need to have unity gain at DC for a lowpass filter?
There is no mathematical explanation and it's typically just a convenience for most applications. If you scale so that it's unity at 0 Hz, unity is also the maximum gain at any other frequency so you are less likely to clip or overdrive something in the signal chain (it's still possible though). 

It's often based on the assumption or requirement that "frequencies below the cutoff should be unaffected" by a low pass filter. The closest way to get this is to have unity gain at DC.