Although the term DNS zone delegation may seem new to you, you have been using zone delegation without even realizing it. When you tell your domain’s registrar what nameservers you are using for a particular domain, you are delegating the zone for that domain.
Single zone delegation will work for most people. In fact, most use a single zone file to reference multiple hosts, or multiple subdomains residing on a single host. However, there are cases where where it may be beneficial to perform additional delegation.