They are great because:
1. You don't have to worry about re-uploading your documents every time you make a mistake. This is because your Google Doc has a URL (a web link), and your students will be assessing the web link. So when you make a change to the document, it automatically changes. And it changes it in real-time. If you are teaching an online class, Google Docs can keep you from going insane.
2. It doesn't matter what type of software your students have; all they need is a web browser to view and download Google Docs. So you don't have to worry about compatibility issues.
3. It is a WONDERFUL place to store your documents. Your documents are stored on your Google Drive, so you don't have to ever worry about losing them.
4. Google Docs are SUPER EASY to create!!
Are you convinced yet? Give it a try!