Pow basically, it is a working mechanism for block attacks and unwanted messages(spam) that will disrupt the working flow of a functioning system or network.
The working mechanism proves that enough energy or effort has been expended for a job to be done.
In the first time used in a article by Cynthia Dwork and Moni Naor and then in 1999 ,was invented a Hashcash mechanism by Adam Back for block DDoS (Denial of Service) attacks and spam messages.
In 2008, a person or a group named Satoshi Nakamoto published an article in which he described the Bitcoin blockchain. In this article, he has adopted the PoW working mechanism to ensure the decentralization and security of the network.

• PoW is a consensus mechanism used by crypto currencies.
• The main purpose of PoW is to guarantee the accuracy of every transaction made to the blockchain and ensure that the transaction takes place securely.

Disadvantages:

• High energy consumption: Adding new blocks to this chain is performed by users who win prizes by solving mathematical problems.
• Slow processing speed: The processing speed may be low because mathematical problems must be solved in order for new blocks to be added.
• Equipment: It is necessary to have powerful equipment to perform these operations, which leads to a shortage of devices.
• %51 attack: For cryptocurrencies with fewer users, there is a possibility that the blockchain will be hijacked by these attacks.

These disadvantages have led to the popularity of other consensus mechanisms such as Proof of Stake (PoS) and Delegated Proof of Stake (DPoS) as alternatives to PoW. However, PoW can still be effective in many situations and some cryptocurrencies still use PoW. Therefore, considering the advantages and disadvantages of PoW, it is important to decide which consensus mechanism to use.