The primary reasons to get a DNS cluster are:

1. You can add as many servers as you want whilst only using a set number of nameservers, e.g. you have 20 hosting servers but all hosting clients use the same ns1/ns2/ns3/ns4 nameservers

2. You can move accounts between hosting servers with zero downtime

3. In the event of a server outage, the DNS service still works and mail is not bounced immediately. And when the server is back online, websites are immediately available.