Discord also has a big lead in video support that, IMO, now exceeds anything else. It has flexible group based video chat that naturally extends voice chat. It has broadcasting support so you can stream your gameplay to each other, as well as player-in-player, pop out support, and side-by-side view. Almost all of these features work on mobile as well, including PIP/broadcasting (e.g. I streamed Genshin Impact directly off my phone.)
These may seem like gimmicks, but I use all these features extensively in a server of ~8 regulars to the point we reflexively broadcast to each other and regularly watch each others gameplay, comment, or just use it to hang out. I think this stuff will probably significantly solidify Discord's lead, especially considering the technical infrastructure necessary for features like this.
Before all that though, the slack-like features, and definitely the moderation features for bigger channels, were - still are! - major attractions.