It's a good idea to use it if you want to see more than just the text of the document (you can do this by choosing "View as HTML") or if you want to edit it, but you don't have an Office suite. The changes will be automatically saved, you'll have access to the previous versions and you can invite your contacts to edit the documents collaboratively. No more sending documents back and forth.
And a bonus tip: if you open the same attachment multiple times, Google Docs will offer you two options: open the existing document or a new copy of the attachment.
