Proof-of-work (PoW) was the first consensus algorithm (it was created for bitcoin). Others have been suggested and some have even been implemented (proof-of-stake is probably the next most well-known). PoW gets bad press because it uses a lot of electricity and therefore isn't "green" (in reality, it incentivizes finding the cheapest electricity that no one else is using... but that's not the point here).
In case you're not familiar, consensus is how blockchains keep everyone honest. It's the piece that determines who gets rewards (miners who add value to the network) and who gets punished (bad actors who try to harm the network).
With PoW, you're typically rewarded with new tokens that the blockchain creates just for you (this is true for bitcoin and for the Koinos ERC-20 token).
The punishment comes in the form of wasted money since you had to spend real money on computer hardware and electricity. If you don't get rewards, you don't recoup your costs.
The fact that PoW requires real money is the key. If you want to create a brand new token, you need some way to distribute it to people. You could sell it in an initial coin offering (ICO), but that often comes with a few investors owning a large percentage of the total (fine in some cases, but increases risk/decreases trust in the network).
That's why Koinos was "fair launched" with a PoW token. Once the mainnet launches, a new more efficient algorithm will be used instead of PoW. That algorithm is being announced on Sunday...
P.S. If you missed it, there's an excellent Coin Telegraph article by Andrew Levine (CEO of Koinos Group) discussing PoW. Check it out!