Answer :
backup is necessary because in case if you upgrade your windows , unfortunatly you files may get erased off , virus may attack your computer so, you can't acces your files in computer , computer can also get dameged so , you can not switch on your computer it self in this cases you need external back up also .
Computer hardware can fail, data can be corrupted, computers can be lost, stolen or destroyed. You may be able to reinstall operating systems and applications but your original data could be lost forever. In these cases Data backup process or software is useful. It protects your data and stores it for our later use.