I feel like “downloading a file removes it from the computer you’re downloading it from” was a weirdly common misconception in the 90s. I’m sure I remember some Star Trek episode that felt the need to specify that a bunch of data had been “copied and downloaded” from the hero ship’s computer so that the audience wouldn’t think that the data was now gone. Maybe the desktop metaphor where files are presented as physical objects that can be moved around contributed to this belief. Maybe also all the anti piracy PSAs that likened downloading music to stealing a car

“Mmm let’s see, the ship is about to go into a potential crisis situation, yes this sounds like the perfect time to break up a ton of working relationships and give my whole crew jetlag”

For me (I use Kavita) it’s because I want to be able to just pick up whatever device is in front of me at the moment and pick up the book where I last left off even if it was on another device