I don't remember exactly when I did this, but I think a friend wanted to do software that would give him money over the internet without having to work. He didn't know how to program. He mentioned this idea he had which was some kind of football championship manager solution, but I heard it like this: a website that generated a round-robin championship table for people to print.
It is actually not obvious to anyone how to do it, it requires an algorithm that people will not reach casually while thinking, and there was no website doing it in Portuguese at the time, so I made this and it worked and it had a couple hundred daily visitors, and it even generated money from Google Ads (not much)!
First it was a Python web app running on Heroku, then Heroku started charging or limiting the amount of free time I could have on their platform, so I migrated it to a static site that ran everything on the client. Since I didn't want to waste my Python code that actually generated the tables I used Brython to run Python on JavaScript, which was an interesting experience.
In hindsight I could have just taken one of the many round-robin JavaScript libraries that exist on NPM, so eventually after a couple of more years I did that.
I also removed Google Ads when Google decided it had so many requirements to send me the money it was impossible, and then the money started to vanished.