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Maybe not strictly necessary, but a separate PCIe backplane just for P2P bandwidth bypasses topology and bottleneck mess[1][2] of PC platform altogether and might be useful. I suspect this was the original premise for NVLink too.

1: https://assets.hardwarezone.com/img/2023/09/pre-meteror-lake...

2: https://www.gigabyte.com/FileUpload/Global/MicroSite/579/inn...