Why do we need AudioFormatReaderSource rather than just use AudioTransportSource to read what AudioFormatReader gets and control its playback?

General JUCE discussion
#1

the AudioFormatReaderSource class for reading and playing audio from AudioFormatReader objects; and the AudioTransportSource class for controlling the playback of an AudioFormatReaderSource object.

#2

The AudioTransportSource can play any AudioSource subclass. But AudioFormatReader is not an AudioSource.
An AudioFormatReaderSource is an AudioSource, that aggregates an AudioFormatReader.