[datpiff]

Add 10ms group delay at 50 Hz to a mix. You can very easily degrade transient response and evaluate the audible difference.

If you want reasoning of the importance, it should be obvious and it's also spelled out in the article

>> Group delay is not some imaginary construct that helps acousticians feel important, it's real — and it means, for the reflex-loaded NS10 option, that a bass-guitar fundamental at 60Hz will arrive at the listening position around 9ms after the second harmonic at 120Hz. Put another way, and expressed as a distance, the low fundamentals of the bass guitar (and parts of the drum kit) will sound as if they are nearly four metres behind the rest of the band (you can insert your own bass player gag here). Low-frequency group delay doesn't only influence mix decisions: it also varies widely between speakers and, unlike low frequency level, which can be adjusted via EQ, once its influence on tracking or mix decision has been 'printed' to the mix, it can't be undone.