It’s being used primarily as a PCIe switch, the utility of which is dynamic bandwidth distribution. To connect NVMe directly, without a switch, requires 4 lanes per blade. Likewise, a SATA controller typically requires at least 1 dedicated lane.

You could use this expansion card to, for example, add switched PCIEe lanes to a mini-ITX mobo via one of its unswitched (direct to CPU) m.2 slots.

What’s unique about this is that aside from the actual total SoC PCIe bandwidth limits and the latency added by any switch, discrete chipset PCIe expansion hypothetically allows an arbitrary amount of extensibility within nearly any footprint.

The likely intended use case is GPU farms where async I/O at x4 to an arbitrary number of PCIe devices can minimize latency between batch cycles up to the capacity of the CPU. The M.2 NVMe slots, SATA and USB might suggest an emphasis on storage and peripherals, but that’s just what the b650 chipset hosts downstream. M.2 Oculink adapters to x16 redriver boards are ubiquitous and cheap, and that plus this expansion card allows AM5 consumer CPUs to be used more like AMD’s enterprise server platforms.

Essentially it turns a pcie X4 slot into 4 sata ports, 4 NVMe slots, and 4 usb ports.