This is the big thing, in my experience.
Stuff that's more immediate and transient ("Hey, anyone want to help me get my first clear of Delubrum before I have to get offline in an hour?" or "Does anyone have <crafting material X> to spare?" or "Anyone want to grind through Wonderous Tails for the week?") is well-suited to the FC channel; if someone logs in an hour later and doesn't see it, that doesn't matter. Stuff that's likely to be relevant longer-term, though? That's worth having on Discord's text channels.
Imagine someone asking "Hey, we're putting together a group for later tonight to get anyone who needs it a Castrum clear; we're going to hit Bozja around 4pm Pacific/7pm Eastern, let me know if you want to come along." If they say that on the /fc chat in-game, those who are online at that exact moment will see it. If that's asked at 2pm Pacific (to allow people time to prep essences or whatever, or to make sure they aren't stuck in the middle of a duty or whatever), someone might be offline because they're at work and not in-game. They could therefore miss seeing it, and thus miss out on the event.
If they ask it on the Discord, though, someone who isn't in-game can see it and go "Oh, I really need my Castrum clear! I'll be home by then, save me a spot." or "I really need my clear, but I won't be home until 4:30pm Pacific. Will that be too late to join in?" or whatever.
This is why stuff like "Hey, anyone want to do a map party this weekend?" or "We're going to put together some alliance raid runs for folks working on their relic tonight, for whoever's interested." or whatever is usually asked on the Discord for my FC; it makes it possible for more people to see it.
If it was always just announced in-game, I think that would be more prone to unintentionally clique-y events, because only people online at the moment of the question -- or the folks those people were friends with and would specifically flag down with "Hey, this is happening this evening if you wanted to join in" -- would have a chance to participate. By organizing events on the Discord text channels, more people have an opportunity to see that they're happening and join in.
But just transient in-game chat? Yeah, there's no reason that shouldn't be on the in-game FC chat channel.