The debt carried when staking fluctuates based on the current value of the issued synths. Imagine a trader making a profit of $50 by going sBTC and then selling to sUSD after sBTC has risen. This $50 profit is "paid" to the trader by issuing debt on all stakers proportionally to their stake. Being a staker is like owning a small part of a B-book exchange. You gain money from fees, from traders who make bad trades, and you lose money from traders who make good trades. Additionally, there's an inflationary SNX reward to mitigate the low volume on the platform since it's not feature-complete. Learn more about SNX staking from their Discord channel.