A testnet is a tool for developers building on a blockchain to test new features, launch a new project, or make an update without the risk of putting it onto the live blockchain (mainnet) and encountering issues. While, in theory, developers may anticipate a certain outcome with their planned updates or features for a blockchain, those may not always go as planned. This is why testnets are essential — they allow for testing without jeopardising the blockchain should they run into unforeseen risks or issues.
Developers typically launch on the testnet to look for bugs and errors; once completed, they can launch onto the mainnet. For example, before Ethereum moved from the Proof of Work (PoW) mechanism to Proof of Stake (PoS) on its blockchain, it first implemented the changes on its testnet Ropsten to ensure everything was in order.
