Can someone explain the "Opening a file" tutorial?

Getting Started
1

I’m new here, trying to understand the “Opening a file” part of the Tutorial: Build an audio player.

I really cannot figure out why are we creating a

auto newSource = std::make_unique<juce::AudioFormatReaderSource> (reader, true);

instead of just using the already existent member

std::unique_ptr<juce::AudioFormatReaderSource> readerSource;

and we pass the reader to readerSource, but two lines later … why the extra step ?

this is the code in the tutorial :

        chooser->launchAsync (chooserFlags, [this] (const juce::FileChooser& fc)     // [8]
        {
            auto file = fc.getResult();

            if (file != juce::File{})                                                // [9]
            {
                auto* reader = formatManager.createReaderFor (file);                 // [10]

                if (reader != nullptr)
                {
                    auto newSource = std::make_unique<juce::AudioFormatReaderSource> (reader, true);   // [11]
                    transportSource.setSource (newSource.get(), 0, nullptr, reader->sampleRate);       // [12]
                    playButton.setEnabled (true);                                                      // [13]
                    readerSource.reset (newSource.release());                                          // [14]
                }
            }
        });

the tutorial explains : We store the [AudioFormatReaderSource] object in a temporary std::unique_ptr object to avoid deleting a previously allocated [AudioFormatReaderSource] prematurely on subsequent commands to open a file.
But I cannot understand how this code is doing something like that if between that line and the releasing that unique_ptr the only thing happening is :

                    transportSource.setSource (newSource.get(), 0, nullptr, reader->sampleRate);       // [12]
                    playButton.setEnabled (true);

is not the same just doing ?:

 chooser->launchAsync (chooserFlags, [this] (const juce::FileChooser& fc)     // [8]
 {
     auto file = fc.getResult();

     if (file != juce::File{})                                                // [9]
     {
         auto* reader = formatManager.createReaderFor (file);                 // [10]

         if (reader != nullptr)
         {
             //auto newSource = std::make_unique<juce::AudioFormatReaderSource>(reader, true);   // [11]
             readerSource.reset();
             readerSource = std::make_unique<juce::AudioFormatReaderSource> (reader, true);   // [11]
             transportSource.setSource (readerSource.get(), 0, nullptr, reader->sampleRate);       // [12]
             playButton.setEnabled (true);                                                      // [13]
             //readerSource.reset (newSource.release());                                          // [14]
         }
     }
 });

This code throws when trying to load a file for the second time. But I don’t understand why.
The documentation says transportSource.setSource() does not delete the source and that it needs to be managed by the caller so it should not be a problem with the code above.