I’ve seen the app Apollo as the center of the reddit protest (it was mentioned and cited more than any other app in relevant posts). I’ve also seen many Lemmy clients in development taking inspiration from it.
As a lifetime Android user I’ve never been able to use it, and I’ve never gotten a proper answer to “why not just use the official app?” What made it different from the official application and other unofficial clients that consequently made it so popular among Redditors?
Apollo was big in the headlines because its developer was the most vocal about the API changes.
As for why people used third-party apps, it’s mostly a preference thing. Something to note is that reddit didn’t always have an official app. Everyone using reddit before 2016 had to use a third-party app if they wanted to use reddit on their phone. A lot of the apps we watched get shut down, especially the ones on android (RiF, Sync, BaconReader, etc) have been around for a long time, and had loyal user bases.
Apollo was younger than the official app, but it was likely favored by those who had used Alien Blue (a very popular third-party app for iOS that was bought by Reddit and turned into the official app)