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).
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).