A13 processor have two USBs, one is HIGH SPEED USB host, the other is USB-OTG.
A13-OLinuXino and A13-OLinuXino-WIFI have build in USB hub, so the USB hosts support keyboards, mouses etc HID devices.
A13-OLinuXino-MICRO have no hub, so on the USB host you can’t attach directly HID devices, only through hub. Interesting feature is that USB-OTG supports HID devices, but to connect USB keyboard or mouse is difficult to USB mini connector, this is where USB-MINI-2-USB-HOST is used.
If you want to connect to A13-OLinuXino-MICRO keyboard you can do it with this adapter connected to the mini USB-OTG connector.