Launching a cryptocurrency project directly on the mainnet without thorough testing on a dedicated testnet can lead to serious and potentially disastrous consequences. The primary importance of having a testnet lies in its role as a controlled environment where developers can assess, refine, and debug the functionality of the blockchain system before exposing it to the broader public on the mainnet.

Firstly, a testnet allows developers to simulate real-world conditions and scenarios, testing the resilience and performance of the cryptocurrency network without risking real assets. This is vital for identifying and rectifying any vulnerabilities, bugs, or potential security breaches that may compromise the integrity of the blockchain.

Secondly, a testnet provides an opportunity to assess the scalability of the cryptocurrency project. By mimicking various transaction volumes and network loads, developers can ensure that the system can handle increased demand without degradation in performance, ensuring a smooth and efficient experience for users.

Moreover, the testnet serves as a platform for developers and users alike to familiarize themselves with the features, interfaces, and intricacies of the cryptocurrency without any real financial risk. It becomes a sandbox for experimentation and learning, fostering community engagement and feedback.

In summary, a testnet is a critical prerequisite for launching a cryptocurrency project on the mainnet, offering developers a controlled environment to refine their code, assess performance under various conditions, and enhance the overall robustness and security of the blockchain system.