A big thing to bear in mind is that Reddit has been around since 2005, but has only had an official app since 2016. For context, the first Facebook app came out in 2008, and while Reddit certainly wasn't that big at the time, it's still weird that it took so long to actually get an official app out there. More over, Reddit didn't start from scratch. They bought up the Alien Blue 3rd party app, which was the top dog on iOS at the time, and reworked it into the official Reddit app.

The point here being that, for a very long time, 3rd party apps were the only reasonable way to browse Reddit on mobile. Even after the official app came out, Reddit themselves stated there was no intent for 3rd party apps to go away. For a lot of us, those 3rd party apps simply were Reddit on mobile.

To more directly answer you question though, I used BaconReader for 11 years on Android. BaconReader still to this day has a slew of customization and tweaking options that the official app just doesn't have. Even simple features, like color coding the left edge of comment blocks, thereby making it much, much easier to keep track of where comment threads diverge. These things also weren't limited to BaconReader, as most 3rd party apps came to the same conclusions: there's pretty simple ways to improve the Reddit experience by just offering a few extra options and thinking a bit about how people use the site.

BaconReader was light, fast, barely used any data, and just worked really well. By contrast, the official app just... doesn't feel all that well put together. The basic usability options everyone else figured out years ago aren't present, the app is more than double the size of what BaconReader was, it positively guzzles data, and while there's lots of updates and feature adds, none of them do much to actually enhance the experience of... you know, browsing and interacting on Reddit. For me, the official app really isn't any better than just using a web browser on my phone, and that certainly uses less data and is less fussy, so that's what I use now. Certainly cuts down on just endless scrolling though.