What exactly are apps used for?

Whatever the user just needs them for.

Apps are shortcuts to programs- games, shopping, information services or whatever. 

If you don't know what apps are used for, that is not a line of work for you.

I'll take the bait. 

In the old days, it was called "software" or "programs." It was/is computer code ("instructions") that did specific tasks or jobs such as business accounting, word processing, document storage, complex math calculations, or controlling robots. 

Now, coding for these and other tasks, including games, is as much about user interface design, user interaction, database design, and "cloud" based computing. It just seemed too old-fashioned and insufficient to continue to call it "programs." 

So, the word "application" or "app" came to be better at describing the "program", the user interface, and everything else involved, as a whole. In other words, the "program" is just one part of an app.

Mostly to mine your system for as much personal information as they can get to use to market products to you in targeted advertising.