There are a surprising number of USB devices that don't work right when plugged into USB 3. I figure it's a compatibility-driven move to support random cheap janky controllers/keyboards/mice that customers will inevitably try to use.
In a mobile device, it's not necessarily about the power delivered over the Vbus wire, but about the energy required for the transceiver and serdes logic in the controller. USB 1.0 "low speed" had a clock of 1.5 MHz, and USB 1.1 "full speed" is clocked at 12 MHz. You can communicate at that frequency with a microcontroller on an ancient process node on a coin cell. USB 2.0 "high speed" runs at 480 MHz. Running a processor at that frequency requires significantly more power, but is not too egregious.
USB 3 is clocked at 5 GHz, which requires more power still. Just having a transceiver capable of that frequency enabled will draw a significant load from a battery, regardless of whether you're using the bus for power delivery.
I meant that there’s probably a usb 2 controller with lower power draw reqs compared to a usb 3 controller, but I now see that the ports are on the dock like others said.
Internally, it might be a single USB 3 interface routed to a hub chip that breaks out one USB 3 and dual USB 2, or maybe even three with one of them going to a GPIO controller or some other random peripheral.
