The Silverlight Streaming service, which Microsoft announced last April when it unveiled Silverlight itself, chiefly does two things for Microsoft: it promotes Silverlight and encourages the spread of the client software, and it showcases what has so far been the technology's major strength: video. The free service from Microsoft gives anyone video streaming and application hosting for up to 10GB storage and 5TB of bandwidth using Silverlight.
Now Microsoft has sweetened the deal by offering revenue sharing via another one of their services, adCenter. Offering revenue sharing is a smart move from Microsoft. It is hard to imagine that they're really expecting the Silverlight Streaming service to be a big money maker on its own -- i.e., Microsoft shouldn't expect it to compete with YouTube or other video CDNs -- but as a way to push the Silverlight technology to the public, it's a smart idea and revenue sharing should attract more developers to it.
If you're a developer looking for a free place to host your streaming videos, and you're choosing between YouTube and Silverlight Streaming, as long as you can keep it under 10GB/5TB, the prospect of revenue share might tip the scales in favor of Microsoft's service.