They get you to sign up, give your credit card, use their nameservers, and then if you get any serious traffic, they will conveniently be there to offer extra features with a paid plan. In my opinion, this is exceptional marketing.

Cloudflare's entire business model is positioning themselves as the middleman to every part of the internet.

They don't host your site, but they'll mirror the front-end on their CDN to speed it up. They'll handle your DNS and TLS certs, and even your domain name, even though they aren't (yet) in the business of selling either.

Pretty much everything Google and Amazon offer as side services Cloudflare offers for free to some extent, and then charges for enhanced versions of those products.