Their token is used as an incentive to correctly decentralize the Tokamak Plasma Chain. The service operator must deposit TON to open the Plasma Chain. If the chain properly runs, this deposit will gradually increase. However, if there is a technical issue in the chain and someone who finds problem raises an issue, and then, the deposit as a reward will go to that person (challenger).
Moreover, TON is also used to charge Stamina, which is the transaction fee in Plasma Chain. The stamina will be recharged after a certain period. It does not disappear until it is withdrawn.