Suppose your social network for chinchilla owners has taken off. Your flagship app contains an embedded chat client, where community members discuss chinchilla-related topics in real-time. As your user base grows, so does its value as a target for advertising. Soon, purveyors of unsolicited advertising take notice of this fact.
You now have a spam problem on your hands, and your small team of engineers has only so much time they can dedicate to this arms race.