The Apple Watch has supported third party apps since it first went on sale on April 24, 2015. Unfortunately, however, because of the limitations of the hardware and the software, usability has been limited. Graham Bower of Cult of Mac wasn't very far off the mark when he wrote an article in 2016 titled "Apple Watch apps kinda suck, but Cupertino hopes you won’t notice."
Fortunately, with the new Apple Watch Series 4 and watchOS 5, I think those days are over. Third party apps which have complications on my watch face or which are stored in my dock now launch pretty much instantly. And just as impressively, even third party apps which I use less often and need to access by pressing the Digital Crown to see all of my apps now launch almost instantly, often under a second. Moreover, with the speed of the Apple Watch Series 4, performance is high enough that apps are much more responsive. As a result, Apple Watch apps no longer "suck," and I'm sure that Cupertino is happy for you to notice that.
All of this means that I'm starting to download more apps for my Apple Watch. Some are more useful than others, but at least now all third party apps have the ability to be really good. Just to name one example, PCalc is a great calculator on the iPhone, but it is also a very usable calculator on the Apple Watch — much better than the Casio Calculator Watch I wore back in the 1980s.
As I have downloaded more apps to my Apple Watch, there are more apps to choose from when I press the Digital Crown on the side of my watch. To make it easier to find the app that I want, I'm now taking advantage of a feature that was introduced in watchOS 4 last year: the ability to switch from grid view to list view. Grid view with its honeycomb layout is pretty, but unless you remember exactly where you placed an app, you will waste time searching around the screen to find it. In list view, everything is alphabetical, and it is quick and easy to spin the Digital Crown to scroll to the name of the app that you want — something which is made even easier with the haptic feedback added to the Digital Crown in the Apple Watch Series 4. You can now feel it as you scroll past every app in the list.
To switch from one view to another, simply press the Digital Crown, and then regardless of whether you are currently in grid view or list view, force press on the center of the screen. This brings up a screen with the option to select either grid or list view.
If you own an Apple Watch Series 4, I encourage you to enable the list view so that it is easier for you take advantage of third party apps, even if you don't use them very often.