The Benefits of Streaming over Progressive Download Flash
Streamed video is more secure than progressive download delivery; you can include encrypted streaming and content verification.
Live video streaming
This cannot be delivered through progressive download; it requires a streaming delivery method.
Multiuser capability and network efficiencies
Allows you the ability to stream live video and audio from one or many users to multiple users, and unlike progressive download, Flash Media Server streaming uses only the bandwidth required, since only the bits that are viewed are transferred to the client over the network. This translates into more clients served for a given amount of outbound bandwidth.
You have the ability to jump into any point within the video and instantly start playing; a spin off of that, is allowing users to select camera angles and video branching, which is useful for switching to alternate endings or other interactive storytelling devices.