Whenever a dark anchor spawns nearby, I always head to it, as does most everyone else in the area. We fight stuff -pew pew! - and then the portal closes.
Why did we do that?
I keep participating in these things because I feel like I'm missing the point, and if I keep participating, I'll discover what it is. But I haven't discovered it so far. I've never gotten any benefit from a dark anchor event, so far as I can tell. Walking across the landscape killing random mobs is more productive in terms of loot. So what's the point in participating in a dark anchor event? I don't get it.