With NodeJS you can build traditional web applications, such as websites. In fact, there are several CMS projects based on Node. However, the most common use of NodeJS is the development of web services that return data in JSON format, which we usually call a REST API. Also with Node we can build multiplatform desktop applications, which have a capacity compatible with Windows, Linux and Mac, as well as console programs. Many day-to-day developer tools are made using NodeJS, from publishers such as Atom or VSCode, to task automaters such as Gulp or asset packers such as Webpack or Rollup.
One of the main characteristics of NodeJS is to undertake many tasks with low resource consumption, which makes it especially interesting for the development of high concurrency services.
But in case you are still not convinced of the importance of Node JS and its service development, here are some of the companies that use it: