Developers worldwide are experimenting with the Ethereum blockchain and many of them find Ethereum testnet hard to use. Because of that, you may want to set up your private blockchain network. You would have full control over that network and would not have any blockchain apps that you don’t need.
So if you want to have an exact blockchain setup only for yourself, you have to define a couple of things. One of them is genesis.json, a file where you determine all specifications about your blockchain’s genesis block.