The Proof of Work concept was invented by Cynthia Dwork and Moni Naor long before the invention of Bitcoin in the year 2008. This concept was initially intended to prevent computers from spam mails. 

The idea was that in order to send an e-mail, the sender have to put some sort of computing power in order to deliver it. As for normal mails this was not an issue, it prevented mass spam mails from being sent as it would have needed massive amounts of computing power to deliver the mails.

Bitcoin was the first cryptocurrency making use of the same principle but in a different context. In terms of a Blockchain the concept is providing efficient resource allocation within the network. For every transaction miners verify, they get a reward in Bitcoin. Transactions can be verified with solving a random algorithmic problem requiring computing power. If you think further, the one with the highest mining power (hash rate) has the highest probability to verify transactions and getting more rewards in return. The proof of work concept is creating incentives that miners supply the network with computing power, keeping the Blockchain self-verifying and safe.

However the Proof of Work concepts has some downsides. All miners providing computing power need enormous amounts of electricity. The current electricity consumption of Bitcoin is estimated to be around the energy demand of Ireland.