Tokens/Coins are used as a medium of exchange between the states. They are digital assets built in to perform a specific function within a blockchain.
When someone does a transaction, there is a change of state, and coins are moved from one address to another address. Apart from that, transactions contain some additional data; this data can be mutated through the change of state. For this reason, blockchains need coins or tokens to incentivize the participants to join their networks.