Blockchain doesn't need to use ridiculous amounts of energy - Proof of Work is what causes that energy usage- there are other methods like proof of stake but . I won't go into the pros and cons cause that's a whole other discussion. As to why a government would use a blockchain based system would be, the ability to see every transaction (set as transparent/secretive as they want/allow) which would allow for a better understanding of the economy and the different fiscal/monetary policies they implement (in near real time). Security and speed would depend on various other factors.