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Raspberry Pi B+ and Raspberry Pi 2/3 (B model) have 4 USB ports on board, which
are usually enough for your project. However, sometimes you may need more USB
ports. Below are some USB devices you may want to connect to your Raspberry Pi:
Keyboard
Mouse
Wi-Fi Dongle
3G Dongle
Flash Drive
Portable Hard
Disk
USB Audio Card
SD Card Reader
Bluetooth
Adapter
Arduino Board
USB Logic
Analyzer
USB Camera
You may not want to connect all of them at a time. The point is that sometimes you
may need more than 4 USB ports. As for the A/B/A+/Zero model and the compute
module with development kit, they have only one or two USB ports and will need a
USB hub when you want to connect more than one USB devices to them.
For different models of Raspberry Pi, this USB hub uses different approaches to
(optionally) back-power the Raspberry Pi, which significantly simplifies your wiring
and allows you to power both the USB hub and Raspberry Pi with only one power
supply.
The unpopulated 5x2 pin header could be used to connect external LEDs for
indicating the port usage state. Or connect to micro controller
for detecting the port’s
state.
