Why Token or Crypto Currency in Block Chain?

Normally any block chain based platform create its own currency or token to give incentive to Miners. For eg: Ethereum being a platform for block chain uses ether as currency. Is it necessary to give incentive to miners in form of crypto currency? Are there any examples of block chain being used without token or crypto currency?

The token is the form of paying miners (or validators in PoS) because it’s a native digital asset,and as such it can be tracked by the blockchain without the need for any interaction with the real world - which would require a trusted third party.

As for the need of the reward, it’s the way originally found by satoshi to align the incentives inside the network and make everyone inclined to act out honestly. Since the reward is ‘trapped’ inside the network, they will have an incentive to want the network to succeed.

However it would be interesting to see a different scheme in place (but i don’t know any).