Curious to know why would anyone want to use this kind of service instead of the wiki / discussion / documentation / issue tracking provided by the project mana

Curious to know why would anyone want to use this kind of service instead of the wiki / discussion / documentation / issue tracking provided by the project management system? I know this service can "integrate" (mostly just links) with other system. I still don't see what the value added of this system can overcome the pain of using two systems. May be mostly for teams do not use project management?