I remember this but never looked into it enough to get what the point is. They still sell it with an M2 chip.

"includes six internal PCIe 4.0 slots for expansion. It does not support discrete GPUs over PCIe." uhhh, so in case people want an AS chip with most stuff soldered on but also really need certain PCIe cards that aren't GPUs?

Audio interfaces, video capture interfaces, network interfaces, etc etc etc.

Would a TB-PCIe enclosure like the Sonnet ones not cut it? I get that it's still easier to have PCIe built in, but that's a big premium to pay.

Those cost hundreds of dollars and often only have 1 slot. The premium can’t be avoided.

Yeah, but it's either that or buying outdated hardware at this point because they don't refresh the Mac Pro. Idk why Apple didn't just make their own expansion add-on.