I noticed the first marketplace link refers to a freely available textbook on openstax (https://openstax.org/ https://openstax.org/). What is the advantage for

I noticed the first marketplace link refers to a freely available textbook on openstax (https://openstax.org/ https://openstax.org/). What is the advantage for me as a student or professor accessing this content through your site when I can download a PDF directly from the source (the openstax site)?