As the name suggests, Testnet is a separate blockchain for developers to test their new features at their own pace without hampering the actual network. It can be risky to add new products and features to the main blockchain network without calculating all the consequences unless they have been aggressively tested in various scenarios. Testnet is used by developers to conduct the same.
Testnet helps them to keep an eye on the errors before those features are actually implemented on the mainnet. Developers also tend to invite a few users to try out the blockchain features and receive constructive feedback. Implementing these changes directly on the main network can cause vulnerabilities to the system, and worst case scenario, it can even get hacked.
Once the feature has been thoroughly tested on the testnet, it is introduced to the main ecosystem. However, in the case of decentralized networks, the community first votes for the change to be implemented. Only then are the features added to the mainnet. Testnet also comes with various simulation tools, allowing the developers to experiment with crypto assets and try out new features without worrying about its repercussions.