Why would anyone want in audio format a book intended to teach reading?
That is easy to answer.
1) Everybody learns differently - some learn best though sound that they’re hearing.
2) Repetition. After reading to a youth for so many times, it could be useful to let the audio do the heavy lifting just by using replay value.
2a) In regard to repetition: it shouldn’t be too difficult to have/use as a tool to replay select minutes 17-23, pause, then play 17-23 again, as many times as is needed. For example. Or whatever segments are necessary/useful in that moment.
There probably are other reasons, but I don’t question that really. I think the whole series should be available and ready for use even if only one single person wants it. That one person enough deserves the choice.