SPS staking is required for tournaments to help keep bots out of the tournaments and encourage investing and / or re-investing in the game. If you've looked at the SPS white paper there are several parts in the where future SPS rewards will be based on staked SPS.
Once the DAO gets going voting power will also be based on staked SPS.
RE: # New Rewards Season 1 - Splinter Stats Ranked Rewards Season 2 Report Card