Comment by pmontra

I think that I open files in very few cases in my job. I read and write PDF, xlsx, csv, yaml and I write docx. Those have their own formats and we use them to communicate with other apps or with users. Everything else goes in a PostgreSQL database or in sqlite3 because of many reasons and among them because of interoperability with other apps and ease of human inspection. A custom file format could be OK for data that only that app must use or for performance reasons, if you know how to make data retrieval performant.