There's also apps you just want for the integration they may provide (for example, a keyboard app) but don't want to install the entire app.

I have the Pocket app installed on my iPhone, because I want to be able to share stuff to Pocket, but I don't ever actually use the Pocket app to read stuff.

Then there are utility apps, such as the app that I used to setup my wifi, that I rarely open but still want to keep around in case something goes wrong.

There's all sorts of reasons to have apps installed but not showing in the most prime real estate in the world.

Nah, I have (for one example) a collection of apps that just exist to power the real version on my watch.

Note that these apps actually do something in some cases, like I have a Twitter app (Chirp) where I'm pretty sure the heavy lifting happens on my phone and the watch just does the display.

Often times the "junk" app is a third party app that the OS allows them to uninstall but they can't uninstall because they need to have it once a year for checking something.