I buy most of my books from Amazon. I do have a Kindle but these days I mostly read on my phone using Moon+ and it's most comfortable with epubs so I use Calibre to convert everything to epub.
Of course there's the safe backup as well. I learned that in the early days of ebooks when Barnes and Noble created an ereader and sold ebooks, only readable in their ereader and you had to have a credit card on file with them because that was their DRM key. Then they announced they were getting out of the business and we had 30 days to download all our books, which I did. Then, a few months later when I changed to a new computer the ereader was no longer available and my books were no longer readable.
I didn't have all that many books and I'd read them already so it wasn't a huge loss for me but it was for some people and it could have been bigger for me if I'd bought more books from them. I learned my lesson.
Every company's TOS says you don't own your books, you only own the right to read them. My TOS says I own my books because I paid for them and Calibre is my hired gun.
Barry