

Xiaomi bootloaders used to work like that. You’d have to jump through some bullshit hoops to register your phone for bootloader unlocking, and then wait a few days to finally be able to unlock it. Then they made that worse and worse, and afaik it’s so insanely difficult and inconvenient right now that it’s practically impossible to unlock your Xiaomi phone’s bootloader. This applies to all the brands under that umbrella.
I am about 97.37% sure Google will do the same over time and at some point you just won’t be able to install any APKs.
Because calibre also allows me to convert other formats into epub.
Some files are unreadable garbage because of bad OCR or bad formatting or whatever. I use calibre to preview files in its built-in viewer, to see how they would be rendered on my actual reader. Helps a ton.
Some files have messed up metadata. Calibre helps with fixing that. I have encountered files that would appear as documents on my Kindle rather than books, for example. Easy fix with calibre.
Even if it is not messed up per se, I still sometimes use calibre to sometimes edit metadata to tidy them up. So that the author information between different books of the same series is the same, for example. “Banks, Iain M.” for all the Culture books, rather than a wild mess of various different variations of the same name. I have also added missing pieces of information to help group books in my library etc.
It’s a super useful tool. I just wish it didn’t spam so many system notifications though.