I recently wrote about how last year it grew twice as fast as I expected: https://maxrozen.com/2023-focus-single-product-pays-off
It all started because I needed a weekly report for my contracting clients to prove their web host sucked to the point where it was costing them significant money. They were paying for cheapest tier WordPress hosting at the time, and didn't believe me when I said random 5 min blocks of downtime throughout the day were adding up.
I built a dirt-simple form that takes a URL and sends a notification when the site goes down/up, with a weekly summary email. Then, I kept adding features every day, responding to customer requests and fixing bugs 2 hours at a time, even after I stopped being a contractor.
Since then I also added status pages (example: https://hackernews.onlineornot.com/) and cron job monitoring, to ensure database backups and whatnot run when I expect.