It's not about how it helps, it's a must for balanced amp, all 4 signals must be separated, each channel is driven separately, you could even destroy some balanced amps by shorting both grounds (depends on the amp). And regarding balanced vs SE amp there is always a difference in clarity and details especially in completely balanced rigs from dac to amp and finally to balanced headphones.
In original denon cable there are 4 on headphones side (so it takes both grounds separately but on the amp side anyway it shorts the grounds on standard mini jack 3 pin (L,R,GND) so it is useless here but you can just cut off mini jack and use 4 wires reterminating it with 4 pin XLR balanced connector using some balanced amp then and drive m400 in completely different way with much better control of the drivers)
In original denon cable there are 4 on headphones side (so it takes both grounds separately but on the amp side anyway it shorts the grounds on standard mini jack 3 pin (L,R,GND) so it is useless here but you can just cut off mini jack and use 4 wires reterminating it with 4 pin XLR balanced connector using some balanced amp then and drive m400 in completely different way with much better control of the drivers)