When you need to run external commands or programs from your Node.js application, you can use the spawn() function from the child_process module. This function allows you to start a new process, execute a command, and interact with its input/output streams. The spawn() function takes at least two arguments: the command to run (as a string), and an array of arguments for that command. You can also pass an optional options object as a third argument to customize the environment or working directory.

The returned object from spawn() is a ChildProcess instance. You can listen to its stdout and stderr streams to capture the output and errors produced by the child process. These streams emit 'data' events whenever new data is available. To know when the process has finished, you can listen for the 'close' event, which provides the exit code of the child process.

Using spawn() is especially useful when you want to handle large amounts of output, because it streams the data instead of buffering it all at once. This makes it more efficient and suitable for real-time data processing or when running commands that produce a lot of output.