USB 3.1 provides up to 900 mA (milliamps) at 5 V (Volts) and can provide efficient power for most USB devices that comply with USB Specifications. *www.usb.org
Some USB peripheral devices might have unique power requirements that exceed the power allotment on an individual USB port.
-Storage devices like external HDD
-Speaker, Mic, Tables, etc....
These devices can require you to connect them to multiple USB ports or can have external power sources that need to be connected for them to function.
Apple devices can request extra power *https://support.apple.com/en-us/HT204377
Apple peripheral devices might request more than 500 mA (Milliamps) at 5 V (Volts) from a port to function or to allow for faster charging. These devices include:
- Apple MacBook Air SuperDrive (when connected to supported computers)