Why do I need to pay Siacoin to start hosting?

As a host, you are required to put up collateral. We do this so that Sia can serve as a high-quality storage network.

Potential payment in Siacoin serves as an incentive to keep your host online, and potential loss of your collateral serves as a disencentive for you to go offline. 

As long as you maintain an uptime of 98% or greater, there is no penalty. Learn more in the hosting handbook: https://forum.sia.tech/topic/1037/the-hosting-handbook