What is the exact meaning of following sentence? And in what situations is such a formation used?
We don’t call it a/ the city of music for nothing.
Answer
Phrases of the type
[Z] [don’t/doesn’t] call [X] [Y] for nothing.
Are typically used as explanations for a particular behaviour or attribute, where the attribute is what X is well known for. For example:
They don’t call Oxford “the city of dreaming spires” for nothing.
Where Z is they, X is Oxford and Y is the city of dreaming spires. The university buildings are grand and have many spires, which is why it got that epithet.
The other type of usage for this construction is, e.g.
They don’t call him “killer” for nothing.
Y, or the epithet, is killer and emphasises what his behaviour is like. Such a remark is probably made after some violent outbust from X.
As to your particular example, it simply means that the city is regarded, at least by the person saying it, as highly musical and that the musicians are talented.
Attribution
Source : Link , Question Author : Suhas , Answer Author : Matt Ellen
