The catch is that you must use Cloudflare's nameservers [0]:
6.1 Nameservers. Registrant agrees to use Cloudflare’s nameservers. REGISTRANT ACKNOWLEDGES AND AGREES THAT IT MAY NOT CHANGE THE NAMESERVERS ON THE REGISTRAR SERVICES, AND THAT IT MUST TRANSFER TO A THIRD PARTY REGISTRAR IF IT WISHES TO CHANGE NAMESERVERS.
People generally use 3rd party Nameservers when their Registrar is unreliable or "old school".
Most dot-com era web companies used Network Solutions back in the day - and many still use them. However, DNS updates sometimes take hours, their interface is cumbersome, etc.
It's become quite common to use someone like Cloudflare for Nameserver/DNS duties (among other things) and keep your registrar where you like.