The watch command causes your system to watch for changes in your source and sass directories. When you save edits, your static site files are re-generated automatically. This allows you to view the changes in your preview local server.
The watch command is automatically run as part of the editing workflow actions that occur when you use the write command. You can use the watch command when you want to manually start watching for file changes without initiating the entire sequence of actions that occur with the write command.