Understanding dispatch_async

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I have question around this code 

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSData* data = [NSData dataWithContentsOfURL: 
      kLatestKivaLoansURL];
    [self performSelectorOnMainThread:@selector(fetchedData:) 
      withObject:data waitUntilDone:YES];
});

The first parameter of this code is 

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

Are we asking this code to perform serial tasks on global queue whose definition itself is that it returns global concurrent queue of a given priority level?

What is advantage of using dispatch_get_global_queue over the main queue?

I am confused. Could you please help me to understand this better.

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