The most common use of Node.js is for building server-side web applications. This is because of its ability to handle many concurrent connections with high throughput, making it well suited for real-time web applications.
For example, a web application built with Node.js could be used to create an online chatroom, where users can post messages in real time. The application would use Node.js to handle all of the incoming and outgoing requests, as well as the messages sent between users.