It lets you see the raw underlying packets like IN, OUT, SETUP, DATA0/1, etc. That capability would be most useful for USB device firmware developers who might want to see these packets to track down an issue with the DATA0/1 toggling or something like that which wouldn’t be visible in a higher-level software trace like USBpcap or usbmon.

Also, it lets you sniff USB traffic of a different host machine, e.g. USB communication between a game console and a USB controller.