I am a long year user of "Remember The Milk" (RTM) to manage all my tasks. Although they do still not have a native app on the iPhone, you are able to use Siri on the Apple Watch to automatically create a task in RTM without touching your iPhone.
This is really something I am using very often throughout the day. Especially as Audi has killed Siri in my A3 if the iPhone is connected via Bluetooth to the car system. Siri on the watch is still working like a charm.