In a multi-chain system, mainchain serves as the backbone that ensures the overall security and consensus mechanisms for all connected chains. It handles the primary transactions and state changes, while sidechains can operate independently, allowing for specialized functionalities and scalability. This architecture improves efficiency and reduces congestion on the main network, facilitating faster transactions and a better user experience. Mainchains are generally more secure but may face scalability issues due to increased transaction volume, necessitating the use of sidechains to offload some of the transactions.