here's a bunch of possible applications:

1. simple note taking. like notepad except you have your notes at a place where you can search through them and even link one from another.
2. second brain. you can watch a video about it but basically to organize your thoughts, record things you learn, make connections between things to have a digital brain you can search or browse through.
3. work or school. notes, to do lists, reminders, links to sources, etc all in one place with references via links
4. journal. pretty straightforward, but you can imagine things you could do if you could link from your journal entry to a website, or another entry, or something from your movie collection.
5. database. eg maybe you have a movie collection and want to document all the details, including which ones you watched, which ones you liked, and what you think about them. you can have a file for each movie but also files for directors, actors, etc that you can link to and from, in which you have info on those, including images, tags for easy search.

so you watched a movie and wonder what other movies you own have the same starring actor: search movie, click link to the star page, check backlinks.

obviously not the best use case because imdb exists but this is personal and could be extrapolated to any collection you have, maybe even all of them. why not have the movie adaptation link to the original book?

TLDR

you can think about it like: imagine if you could make a bunch of wiki pages. the formatting isn't quite as nice but essentially that's what you're doing. a bunch of pages with text, images, links and tags, that you can browse through. what would you use it for?