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Dammit, the video doesn’t explain why we have 4 twisted pairs in our cables instead of just 2! One for Rx, one for Tx, but then why did they keep the other two? Surely they didn’t have 4 dead wires just because of legacy? So why the additional pairs, what were they used for?

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Originally it was more or less one for POTS, one Rx, one Tx and one spare (used for power in ISDN S/T interface, which otherwise shares the same pinout).

Then came 1000-base-T, where the “base” is kind of misnomer and it is actually related to SHDSL. Each pair is essentially a separate full-duplex 250Mbps link with active echo cancellation and four of them is combined to create one 1Gbps link.

Signal integrity basically. Using more pairs means reducing the bandwidth needed on each pair, thus the cost of the cable.

It's also a tradeoff with the cost of the NIC hardware --- look at how much bandwidth xDSL technologies have been able to squeeze into a single traditional twisted-pair phone line, but they use far more complex signal processing techniques than Ethernet.

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