I convert most of my books to MOBI, even though I sideload and never send anything to the cloud. For a simple reason - I want to get rid of original formatting (fonts, margins, line-spacing etc.) Converting to MOBI does it faster than editing each book manually would. Of course I edit manually too, when necessary (converting doesn't change everything), but it would be a major blow for me if one day I couldn't convert to MOBI anymore.
Yes, I know I'm an odd bird.
Most people want to retain the original formatting as closely as possible, not to get rid of it. I want all my books to look exactly the same, so I can concentrate on the story and not be distracted by different layouts.