CDNs are a bad example, because they usually deal with DNS as well. They usually want to send different replies in different regions, etc. CloudFlare is the onl

CDNs are a bad example, because they usually deal with DNS as well. They usually want to send different replies in different regions, etc. CloudFlare is the only one I used, but I know you can't set it up before you switch to their name servers. However say you host your root-name website on GitHub pages or similar. You don't want them to have full DNS control over the rest of your zone (emails, app, etc).