I've tried a few shopping list apps for the iPhone but they never really worked for me because it's just too slow to add things. Get the phone out, unlock it, launch the app, press some plus button somewhere... And then even the act of typing is slower than jotting it down with a pen. It was never something that worked for me, personally.
But... Imagine finding out you're out of soap (for example), raising your wrist and saying "Hey, Siri. Add soap to my shopping list".
(Or, a little more clumsily, something like "Hey, Siri. Add soap to my shopping list in To Do Today".)
The Apple Watch is superb at quick interactions based on triggers from the cloud - messages, emails, reminders and such like. It also has a powerful and nearly effortless user interface in Siri which, for reasons I've yet to see an explanation for, seems to work better than on the iPhone. However, Siri is limited to Apple's own in-built services.
I'm convinced that the killer feature for the Apple Watch is the Siri API developers have no access to.