Propagation — resolvers can disagree
A DNS change does not reach the world at once. Each resolver keeps its previous answer until that copy expires, then fetches a new one. This check asks Google, Cloudflare, Quad9 and KT separately and shows whether they agree. If they do not, the change is still spreading.
CAA — restrict who can issue certificates
Without a CAA record, any certificate authority in the world may issue a certificate for your domain name. One record limits that to the authorities you name, which narrows the room for mis-issuance.
DNSSEC — signatures against tampering
DNSSEC signs DNS answers so that tampering in transit can be detected. It is not mandatory, but most registrars now enable it with a single switch.