I understand with an Apple Watch or any other smartwatch it's not really efficient to have full working capabilities.
That's why, you do your work on an computer or tablet, and to be able to trade with your smartphone, is an extra bonus.
You can do most of you work on your phone, if you are on the road.. but while driving, in a meeting, exercising, an notification/ overview of your
positions would be very helpful, and close if you desired.
An Apple Watch is an notification device, so to be able to view and close your positions would very useful.
If SIRI was more advanced you could use SIRI but we're not there yet.