The most popular use case for an external HDD is for backing up data. No matter how reliable an internal hard drive is, it can fail at any time. This is a reality for any component that is constantly in use. That’s why many users consistently back up important files on an external hard drive, which is used rarely. Additionally, external hard disks are not usually constantly attached to a system. This protects the backed up data from damage if the main system suffers an electrical overload, like from a power surge.
The third of the big three things to consider when looking for an external HDD is the interface. Many external hard drives will have either a USB 2.0 or the much faster USB 3.0 connection that simply plug into the interface that every computer will have.