If there's a great Apple Watch companion app for one of your favorite iPhone apps but it isn't installed on your Apple Watch, that doesn't do you much good. With a quick settings change, however, you can configure your Apple Watch to automatically install all companion apps for apps that are on your phone.
By default, the Apple Watch doesn't automatically grab all companion apps---and with good reason. Both Series 1 and Series 2 of the watch only have 8GB of internal storage, a good chunk of which is chewed up by the watchOS and core applications. While that space-conserving default is great for preventing you from getting into a storage-checking and app-deleting loop, it also means you might miss out on cool companion apps you didn't even realize were available.
To that end, it's handy to throw space-conservation to the wind and set your watch to automatically snatch up all available companion apps. After all, you didn't get a smart watch just for text message notifications on your wrist, right? You might as well be sure you get the best of everything it has to offer.
You can turn on automatic app downloading by simply opening up the Watch app on your iPhone. Select "General" from the main screen to get started.
Within the General menu, simply toggle "Automatic App Install" on.
Not only will this enable automatic installation of all future companion apps, but it will retroactively install the apps you might have missed when manually installing them in the past; if you look at the list of applications on the main screen of the Watch app, as seen below, after enabling the setting you'll see the missing apps installing to your Apple Watch.
That's all there is to it! From now on you'll never wonder if there is a companion app for your iPhone apps as they'll already be ready and waiting on your Apple Watch.