First on the list of bonuses for the Twitch and Steam partnership comes in the form of Dota 2. Players will be eligible to receive tournament item drops while spectating the games on Twitch as long as you have a ticket for that event. This used to only be possible through the in-game spectating feature.
From the Dota 2 Blog:
We realize that people have different reasons why they prefer to watch in-game or via streams and wanted to bridge the gap of features they were missing out on by choosing the service that works for them the most. We hope to expand on the features available via this system and would love to hear what you think we should work on next.
Just to be clear, this partnership is not currently about streaming games you play on Steam. This is just allowing you to connect your Steam profile to your Twitch account. I definitely look forward to future interactions between the two services.