Birdfeeders and watering holes are a couple examples of places animals gravitate to in large numbers and great variety. In open world games, points like this can be useful.
One reason is that areas of enemy/prey congregation offer players set points to find content quickly, easily, and repeatedly.
Another reason is that engaging NPCs in a crowd is different than doing so while they're alone or in small groups. The Commonlands savanna outside of Freeport in Everquest 2 is a good example. Hunting one hippo is difficult when another could wander up behind you, lions are roaming nearby, and snakes are slithering hidden in the high grass.
I said "engaging NPCs" instead of "fighting" because congregation points could affect gameplay in non-combat scenarios as well. One NPC could overhear your conversation with another NPC and respond somehow, if programmed to do so.
Congregation points can also serve to place opposing forces together for potential conflict.