I've found that converting EPUB to MOBI is often necessary because MOBI is the native format for Kindle devices, which dominate the e-reader market. EPUB is more versatile and widely used elsewhere, but Kindles don't support it natively. Converting ensures seamless reading without compatibility issues.
Another reason is formatting. MOBI files handle Kindle's unique features like annotations, bookmarks, and font adjustments better. EPUBs might lose some of these elements during conversion, but tools like Calibre make the process smooth. Plus, MOBI files are optimized for slower e-reader processors, making page turns and navigation faster.
Lastly, Amazon's ecosystem heavily favors MOBI. If you sideload EPUBs, you miss out on features like Whispersync. Converting gives you the best of both worlds—accessibility and functionality.