Online backup is an essential hedge against physical data disaster. Whether it's an act of theft or an act of God (earthquakes, floods, hard-drive devouring locusts), you never know what could hit your PC or phone, and when. This is especially important for mobile-centric users, who stand to lose a lot of digital stuff if their device disappears.
Uploading your data to an online service covers a lot of the bases. Some people call these "cloud" services, but in reality, your data is stored on servers at the other end of the connection—that is, someone else’s hard drives. But that's part of the plan! The backup Rule of Three says you should maintain three copies of your data: the original, a copy, and a copy of the copy. One copy should be in a safe place away from the original and secondary.