There are others, but I think the craziest thing about Cloudflare is its basic level of protection is free. Free, unmetered, DDOS protection. It's so popular because so many hobbyists use it for free, and are familiar with it. Then they convince their workplaces to adopt it when the need arises because they are already familiar with it.
They make money by selling support to companies, and selling access to some more advanced features (that often have a free tier as well). It's honestly so impressive, it made me wonder how much they actually make because it seems unnecessary for most to pay at all. Turns out they cleared almost a billion dollars in revenue in 2022.