It's a fact of life, computers do crash. Or they get attacked by a malignant software or a virus.
Now, imagine you are working on a dissertation, written by a Mexican student in Spanish. She is applying for a scholarship at a renowned English university to do a master, and you are translating her work into English.
There are a total of 60 pages, you are halfway through and 'bang' - your computer crashes. AND you forgot to back up your files...........
You don't want that to happen to you, believe me. You might have lost there 20 hours of work, and you have to start all over.
So always make copies of your documents (the originals and the translations), and make them OUTSIDE your PC. What I mean is keep those copies on a floppy disc or a USB stick. That way if your computer does crash it at least doesn't make you lose all the time and effort you have invested until that point.