Backup DNS servers (VPS) for our dedicated servers?

i have heard that many people are using VPS only as "backup
DNS servers". I dont get it. How does it helps to our dedicated
servers? for example one of my dedis have their nameservers set to
ns1.myprovider.com and ns2.myprovider.com how a VPS acting as
"backup DNS server" will be of help to my dedicated