Why Proof of Work is required in Blockchain?

It is complex, expensive, and time-consuming to establish proof of work verification, but easy to verify. Bitcoin is stable since the network can not be attacked computer-wise. It is fundamental to this property to require evidence of work for participation. As the basis for trust, Bitcoin therefore relies on computational work on cryptographic challenges.

Users broadcast transactions to the network and, however, those transactions are not deemed legitimate immediately. When they are added to the blockchain, it just happens.

The blockchain is a large database that any user can see, so they can verify if funds have been spent previously. Picture it like this: for you and three colleagues, there is a notepad. Whenever one of you needs to move whatever units you are using, you write it down. Alice is paying Bob five units, Bob is paying Carol two units, etc.

You refer to the transaction from which the funds originated each time you make a transaction. Another difficulty is present here. So if Bob was paying Carol two units, the entry would probably look like the following: from this previous Alice transaction, Bob is paying Carol two units.

For units to be monitored, we have a way. If Bob wants to make another transaction using the same units he just sent to Carol, everyone will know right away. The community would not allow the transaction to be added to the notepad.

Now, this could fit well in a small group. Everyone knows each other, so they'll probably agree on which friends to add transactions to the notepad. What if we want a participant group of 10,000? The definition of the notepad does not scale well, so no one wants to trust it to be regulated by an outsider.

This is where proof of work comes in. This suggests that consumers should not spend money that they are not entitled to spend. A PoW algorithm allows anyone to use a combination of game theory and cryptography to change the blockchain according to the system's rules.