Comment by AdamJacobMuller

It's not about proving ownership, if it was about proving ownership we would do this via something at the registrar level.

It's about proving /control/. If a domain name is pointed to me (my IP/CNAME) I control it and it is reasonable to allow that person to issue an SSL certificate for a domain (or subdomain) under their control. If you, as the domain owner, want to restrict that, CAA exists as your tool to do so.