secondary DNS server is meaningful?

[SB CH]

Hello all.

I would like to know the rule of the secondary DNS.
secondary DNS is just backup? 
so it is useless to operate secondary DNS if primary DNS is operated well? 
But some case, a domain is not resolved properly when setup at just primary 
DNS server.
So I guess that the primary and secondary is important alike, right?
I woule like to know the real role of the secondary DNS.
just backup or not?

Thanks in advance.
 

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