During the development phase, you will need to recompile a SASS file each time it changes. Instead of manually issuing the compile command after saving your source file, it sure would be nice to have your computer automatically perform the compile whenever a source file has been modfied. That is what a File System Watcher does. There are a few in use.
I suspect that most Grunt builds are using the plugin grunt-contrib-watch. I'm not sure if you have to install anything besides the Plugin.
The EmberJS project uses Watchman.js, which was developed by Facebook. It is suppose to handle really large projects. It can be installed via npm.