The cluster module allows one to start the same application multiple times.
Clustering is desirable when the different instances have the same flow of execution and don't depend on one another. In this scenario, you have one master that can start forks and the forks (or children). The children work independently and have their one space of Ram and Event Loop.
Setting up clusters can be beneficial for websites / APIs. Any thread can serve any customer, as it doesn't depend on other threads. A Database (like Redis) would be used to share Cookies, as variables can't be shared! between the threads.