Mainly 2 things, which I am now doing using a thunderbolt 1 station:USB will be bottlenecked by the bandwidth needed to run the display anyway. What do you need full speed I/O for?
Gigabit ethernet, for internet and especially for transfers to/from the NAS, including time machine
USB 3 ports for plugging external HDD.
So I don't know what speed to expect from the USB3 ports on a 5K TB3 display, but I am thinking I can expect more than USB2 speeds, right?
USB2 is awful for everything expect little stuff like keyboard etc, which I use BT anyway.