Brownie - lets you branch the head off any EVM blockchain so that you have the current state of all smart contracts and accounts locally, and so you can manipulate the states for free, and if you find a state you like then you switch back to the mainnet blockchain and only pay for that transaction to get the desired outcome you want. also the offline version can just be for educational purposes to understand how and why people developed things a certain way.

For many, this is better than simply spinning up a localhost Ethereum network as offered by Ganache, because those lack any data to manipulate.