First, the instant purchase ability is extremely important. It means that books gain some measure of equality with other forms of media (the internet, movies/tv, music). With a Kindle I can read whatever book I want whenever I want, which means that I read many more books than I used to.
Second, the mobility advantage isn't slight at all, especially for people who read a lot. If you're never carrying more than a single book around that's one thing, but I read 1-2 books a week, and a lot more (often 5+ a week) if I'm traveling. Even just during my daily commute, it's far easier to read while standing on the subway with a Kindle instead of a book.
Beyond that, I find reading on a Kindle to be far more comfortable than reading a book. The actual reading experience is much nicer than trade paperbacks, which are printed very cheaply. It's much lighter and only requires one hand to hold/turn pages, so I can more easily read while lying down, and the newest version has a very nice backlight, so that I can read in bad lighting.
There are obvious disadvantages, and I absolutely hate that DRM is still a thing, but the benefits absolutely outweigh any possible disadvantages for me. Anecdotally, it's the difference between me reading 5-10 books a year and me reading 50-100 books a year, and I'm a much happier person in the latter state.
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Also, if you're not saying something to be rude, you should actually write like that, not add a disclaimer at the top of what you're writing. There were a lot of ways to get the same message across without calling people idiots.