POLK is used as a curation mechanism and a settlement & dispute system in the protocol - where you are able to perform:
Prediction Markets Creation: POLK is required for users to open new markets on Polkamarkets. If you want to know the future and receive the wisdom of the crowd, you must use POLK.
Oracle Dispute: POLK is used in a bonding system that leverages the wisdom of the crowd to arrive at a truthful event resolution based on distributed consensus. You can read more about how market resolution works here.