>>282229752
To have a reliable (i.e., not-fucked-with) overview of the events, to quickly contact specific people from the thread, to talk about off-topic things that don't belong into the thread, to voice chat while playing singleplayer games or doing Blender/Unity et al., to ping groups of people for events/lobbies.

You can do all of that while still having 100% of your own on-topic discussion anonymously in the thread, which I prefer because discussion on 4chan is nicely structured. With the vanilla post numbers and 4chan X inline-linking, you can immediately see which posts are replies to what, and easily follow even a very long thread of discussions from the start to the end, even if you actually weren't there towards the start. It's just the best format of discussion. 
It would be even if we didn't have anonymity, which, however, is also boon by reducing stress due to the low consequences because your identity is limited to usually one post, if you want. 
It also prevents people from changing their stance towards you based on what you said or did before, classic counter example being people siding with the moderator with 20.369 posts on a normal forum because he exudes authority through that, despite you actually having made the better points. Here, ideally people reply based on the merits of your post's content only. Of course, it's not just a boon, but also a bane, because you not knowing whom you're talking to removes context clues that could've made for better discussion more quickly, and anonymous people will make assumptions about their interlocutor along their perceived stereotypes anyways. In addition to that, due to there not reliably enough being sanctions for shitposting, it allows for people to permanently shitpost, wasting the time of people who argue in good faith.

Discord can do none of that, while 4chan can do none of the things in the first paragraph. That's why some people use both and it's mean to take away people's choice to do that.