Because Google Docs saves to a secure, online storage facility, you can create spreadsheets without the need to save to your local hard drive. You can also access your spreadsheets (and all of your other docs and files) from any computer by signing into your Google Account. In the event of a local hard drive crash, you won't lose your saved content.
Since Google Docs continuously saves your work, there's no Save button or menu item. The last updated time shows near the menu bar.