Given so many devices now power themselves using a USB connection, it's important to know how much power the USB port on your laptop can supply. If a device tries to draw more power than the port can supply, you risk killing the port---or even starting an electrical fire.
Not all USB ports are created equal. Your machine might have a mix of USB 2.0 and USB 3.0 connections, each of which might have different outputs. Knowing a USB plug's power output also helps to determine if there are any problems with your ports. For example, if a port is not charging gadgets as quickly as normal, you might discover its power supply is not as high as it should be.