The REN token is used as a bond to run a Darknode, which powers the sMPC network (RenVM).
The decentralized network of Darknodes is permissionless, but to prevent the forging of a large number of identities a good behavior a bond of 100,000 REN tokens is required in order to register and run a Darknode. This prevent malicious adversaries from running an unbounded number of Darknodes and overwhelming the network with misbehaving Darknodes.
This is similar to the way that Proof of Work uses computational work to restrict block production, or the way that Proof of Stake uses staked tokens to restrict block production.
For every Darknode that you register, you will need to acquire 100,000 REN and be prepared to bond it into an Ethereum smart contract. This bond will be returned after the Darknode is deregistered.
Darknodes (and those who run them) earn rewards (in BTC, ETH, ZEC, DAI, and ERC-20) for contributing to the network.