Cynical Joker

You don't need two PC's to stream to be a successful streamer, but it does have 2 big advantages:

1. Streaming is very intensive and the processing requires a lot of RAM and CPU power. When you offload the streaming to a 2nd PC, it means your gaming PC isn't affected by the streaming processing.
2. Your stream stays live when something happens on your gaming PC, like a program or game that locks up, an unexpected Windows update or a blue screen of death. These annoying interruptions happen more than you think and it's very annoying when it takes down your stream too.