When working on a document, you are working in the computer's Random Access Memory (RAM). Though very quick, RAM is a volatile memory. If you don't save your document to the internal hard drive, or to an external storage device, before turning off the computer, all is lost - the document no longer exists.If working on an important document, save to the hard drive every few paragraphs. Then, if things go wrong, you only need to begin again at your last save, and not from the beginning again. Save as you go is good advice.