As someone who has occasionally wished for a book to be adapted into a movie/miniseries/series: sometimes, for me, anyway, it's a matter of wanting to see the story that you're picturing in your head "brought to life". If you enjoy the story, or even the world created in the story, sometimes it's fun to experience it in different formats, even if it's not the same. Whether it ends up being good or not is another thing, of course.
Exactly. It's just another way of enjoying a favorite story or character or universe, like reading a novel based on a movie or a movie based on a book, along with the comic-book adaptation, the live stage version, the video game, or the eventual Broadway musical version . . . .

Granted, some books seem to cry out to be filmed more than others. Certainly, I've had the experience of thinking that a particular book would lend itself well to film (or not), maybe even indulged in some fan-casting: "You know, Zendaya would be perfect as the female lead. Or maybe Florence Pugh?"