Great question! BTW this is Jieyi. I am the CTO of Theta/SLIVER.tv

The main purpose of using blockchain is to encourage viewers to share their redundant bandwidth resource to relay the video stream. In return they'll get Theta tokens as rewards.

The viewers that share their bandwidth resource are called caching nodes. They relay video streams while mining new blocks just as Bitcoin miners. Yet in Theta, the reward for a block is NOT a constant. The more video stream the caching node relayed, the higher block reward it would get. Here is how it works -- when a caching node mines a new Theta block it calculates a reputation score for itself. The reputation score is a measure of the amount of video stream data it relayed in a recent time window. A high reputation score increases the block reward, and in turn encourages the caching node to relay more video. Furthermore, when a new block is mined, all caching nodes must verify the reputation score of the caching node that mined the block, and thus form a global consensus on the block reward.

The calculation of the reputation score is based on the "service certificates" the caching node collected from its downstream viewers while relaying the video streams. The "service certificates" requires hashing power to generate and therefore hard to fake. Yet the "service certificates" are easy for other miners to verify, which makes the global consensus on the block reward possible. To get more details, please look at our whitepaper and the technical overview:

Whitepaper: https://s3.us-east-2.amazonaws.com/assets.thetatoken.org/Theta-white-paper-latest.pdf
Technical overview: https://s3.us-east-2.amazonaws.com/assets.thetatoken.org/Theta-tech-overview-latest.pdf