Sync gained a big following because it had a core feature many many moons ago, when having constant cell service was much less common, it allowed you download and save hundreds of posts and all the comments while you were on wifi, so you could browse reddit offline, it did this automatically and in the background (based on your settings ) hence the name 'Sync'. This was a killer feature back in the day, at least for me. As that became less of a need, the app continued to change and add a lot of nice features, like lots of customizations, random NSFW, a very good OLED dark mode, etc, so there was no point in switching to something else.

My .02¢