Buying your own domain often includes DNS hosting but that’s not really the point unless all you’re doing is exclusively running an externally-facing website or e-mail. The main reason for buying a domain online is so everybody else recognises you control that namespace. As a bonus, it means you can get globally-cognised SSL certificates which means you no longer have you manage your own CA and add it’s root to all the devices which wish to access your services securely. It’s also worth noting that you cannot rely on external DNS servers for entries that point to private IPs, because some DNS servers block that.