There are a billion webpages, and many of them are interactive in some way - in fact, many of them are interactive in the same ways. How many websites have you seen that use a slideshow? I've seen hundreds, myself!
When a programmer creates a JS library and puts it out into the world, they're purposefully deciding to share it with the world - so that often means they put in the effort to come up with great documentation and examples. Perhaps one day you will decide to create a library of some functionality that you find really useful and share it with the world.
But first, how do you actually use a library? I'll show you that next!