I guess it could be seen as a relic considering most registrars/cdns/hosts are a "full package" now. In the days of old that wasn't exactly the case; where you bought your domain name, where you controlled your dns, and where your systems lived could all be different entities. Nowadays there are still reasons to do so...more control and less centralization are some. Not everyone wants to be under the thumb of the Amazons/Googles/Cloudflares.