[kinghajj]

It is, yes. Curiously though, making the work "useful" might actually harm the security of a block chain and make it more likely to suffer a 51% attack.

With a chain like Bitcoin's, if you want to perform a 51% attack, you would have to purchase a large amount of SHA-256 ASIC mining hardware. After the attack, public confidence would be shaken so severely that the market price would crash, and might never recover. So afterwards, the attacker is left with a pile of ASIC hardware that's nowhere near as valuable as before.

If the PoW algorithm were based on something more "useful", then after a 51% attack, the attacker would have a large amount of hardware that they could easily repurpose, so it would retain a lot of its previous value. That makes the total cost of the attack much lower.