Why use async HTTP request over sync HTTP in a separate thread?

I know about the difference between how each works but i want to know in a performance wise point of view (resources inside the iphone).

Lets say I send an asynch request and wait for the delegate to be called. This won't lock my execution thread. But what is the difference of doing this against just sending a synch request in another thread with GCD.

Like this:

dispatch_queue_t findPicsQueue;

findPicsQueue = dispatch_queue_create("FindPicsQueue", NULL);


dispatch_async(findPicsQueue, ^{

    NSData *theResponse = [NSURLConnection sendSynchronousRequest:theRequest
                                                        returningResponse:&response
                                                                    error:&error];

    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;

    if (error) {
       NSLog(@"Error: %@",error)
    }

    if (httpResponse.statusCode == 200) 
    {
        [self parseXMLFile:theResponse]; // Parses Data and modifies picturesFound

        for (PictureData *tmp in picturesFound) {
            NSLog(@"%@",tmp);
        }          
    }  
}

It wont lock my interface since its not being executed in the main thread, but it will lock this specific thread. And I also think GCD runs queues concurrently.

Thanks in advance. I really want to clarify this question.