The separation of security and governance from the stakeholders is a significant one.
On networks where stake and governance are intertwined, attackers can buy stake and attack the network risk-free up to a certain point. However with something like POW mining, the profits are razor thin. Trying to attack the network usually fails, AND it costs a shit ton of money. Even in the event of success they've probably still lost money. On a proof of stake network we've already seen that a hostile takeover requires forking out attacking stake, which is bad policy and seen as unacceptable to a network like BTC.

Again, the ability to turn energy into money while also securing the network isn't "pointless computation".
It's magic.
It's an extremely important tool moving forward.
And it creates unhackable random numbers that no one is even using yet.