High availability significantly increases the range of use cases for blockchain, to include highly important uses such as buying and selling, currency exchange ( needs to be near-instantaneous) and informational uses where high availability reduces response times to queries or completion times to tasks.
Using bitcoin as an example, outside of a complete failure of world electricity or a complete breakdown of the Internet, the incentive to miners to keep the blockchain available ensures that the system stays available. If a part of the network fails, then if there are fewer miners, mining becomes more profitable. In response to that, more people will mine, or mine more, and the system will return to equilibrium where mining is just profitable enough to enough miners to meet the demand to process blockchain transactions. The high availability of the system will thus be ,maintained.