What are the top uses of PCIe x1 Slots?

I have a WD black SN850 500GB in my _1 slot on my ASUS ROG Strix x570i mobo The only thing I can find in bios is set to _1 link = Gen 4

2 slot is connected to CPU and supports the bandwidth of PCIe x4 The second and third slot can either

slot machine play free 3 slots to see if there is any performance difference when switch between slots And, I also

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