I think the intent of a board like this is to serve as a means of prototyping devices. The designer doesn't know what the Hawkboard customers will try to build in the long run, so he provides multiple different ports to maximize the utility of the board for prototyping.

In regard to USB3 specifically, it's plenty fast, but it has the major distinction versus Ethernet that it's not bus-addressable, i.e. it has no provisions for network routing. Also, I suspect (but I'm not certain) that its latency is relatively high compared to something like DisplayPort.