A battery backup, or uninterruptible power supply (UPS), is primarily used to provide a backup power source to the parts in the computer case, the monitor, and any other device plugged in to the battery backup.
UPS, uninterruptible power supply, uninterruptible power source
In addition to acting as a backup when the power goes out, most battery backup devices also act as power "conditioners" by ensuring that the electricity flowing to your computer and accessories is free from drops or surges. If a computer is not receiving a consistent flow of electricity, damage can and often does occur.
While a UPS system is not a required pieces of a complete computer system, including one as part of yours is always recommended. The need for a reliable supply of electricity is often overlooked.