Testnets are test blockchain networks that act and perform similarly to the main networks (mainnets) they are associated with. Since they operate on separate ledgers from the mainnet, the coins on a testnet have no connection to transactions and value on the mainnet. This allows developers to deploy, test, and execute their projects on a functioning blockchain freely.

The cost of failure or mistakes on a mainnet is very high and at times irreversible so developers need an environment where they can comfortably work without these risks. 

Security is a high priority for developers working on any decentralized platform. Operating on a testnet also offers the benefit of both investigating any vulnerabilities in the platform as well as educating developers on how the platform works. For testnets that are for pre-network upgrades, it is important to investigate how newly developed changes function before they are merged with the mainnet.