I don't know where this goes
It's a passthrough and doesn't connect to anything, yet.
This is there for the reason of when you need to use internal USB 2.0 header and you're one short. So this passthrough gives you the needed connector, while still being able to use front I/O USB ports.
As of devices that use internal USB 2.0 header; Corsair Commander Pro, NZXT HUE+, almost all AIOs, among others.
For example; in my Skylake and Haswell builds (full specs with pics in my sig), i'm short of internal USB 2.0 headers and due to this, both builds have internal USB 2.0 header hub in use. This one: