Long story short, I bought some e-books from Amazon in the early 2000s, thinking I can count on Amazon to always have the content available one way or another. But last year when I tried re-downloading the books, I couldn't find them anymore. I contact Amazon support and they basically (politely) tell me to fuck off - they tell me I should have downloaded the files when it was still possible (the rep also tried to sell me a story about how I bought a one time download because the word "download" was in the product title). Amazon never announced phasing out those e-books and never contacted owners when they switched to the Kindle system, so I imagine they won't bother with Kindle era books either.
Basically I want to be able to move books I have saved on the subscription app Scribd onto my kindle e-reader. Mostly to take advantage of the e-ink being better on the eyes than the glare of a tablet screen.