^ its like asking why water is wet.
a blockchain needs to be verified as there is no central authority running it to guarantee its validity. the work of verification is rewarded by fees to the verifiers, which is the currency of the BC.
a BC cant be run without its currency, the value of the currency depends on the immutability and transparency of the BC, and its utilities. eg apps running off it or if it has a specialised application.
if more than 50% of the verifiers are united, the BC validity can no longer guaranteed, as they have the power to change the transactions that have already taken place, and so the currency becomes worthless.
a BC has to be decentralised and permissionless, otherwise it is useless. if it is centralised and controlled it is nothing more than a slow DB.