From Adobe.

Streaming video with Flash Communication Server should be used in any situation where you want or need to do the following:

* Deliver long files—for example, video clips over 10 minutes long
* Deliver video to many viewers—for example, a hundred or more simultaneous viewers
* Have advanced features such as bandwidth detection or quality-of-service monitoring for delivering the best possible experience
* Provide interactivity to your video experiences
* Stream live video
* Capture video

In short, if your website relies heavily on video for delivering your message you will want to present it in the best possible manner and use Flash Communication Server.