Portability: They are essentially portable batteries that can be carried around easily.
Charging on the go: They eliminate the need to search for an electrical outlet when your device's battery is low.
Universal compatibility: They often have USB ports to charge various mobile devices.
Capacity: They come in various capacities (measured in mAh) which determine how many times a device can be charged.
Fast charging: As indicated by the text on the box, many modern power banks offer fast charging capabilities.