I don’t use one third party app on my Apple Watch, not one.
The reason is simple in that I have not found one that is remotely useful to me and the situation works for me. The fitness tracking, notifications and accurate time are all that I want to be honest and I can think of no other use that would make the experience better on the Apple Watch than on the iPhone.
There are times when it is more convenient to make a call from the watch or even send a very quick message, but these are embedded features so again no need for a third party app.
The comparison to my iPhone usage is stark with third party apps dominating my usage so it can only be down to duplication and the minimal form of smart watches.
I have no doubt that there are apps, such as Uber, that will be useful for individuals, but for the majority I suspect that third party apps are nothing more than a novelty that do not get used often. Feel free to correct me if your experience is different.
It leads me to believe that the future of smart watches is in making the features they are good for better and to not aim to make them mini replacements for phones. As an add-on they have potential, as a full-blown mobile experience they can only bring confusion and wasted time. A watch is just a watch after all, no matter how smart it is.