On devices without a full-size USB port (read: most), using external USB peripherals - like keyboards, mice, game controllers, etc. - is basically a non-experience. In order to use said accessories, you need an OTG cable, which essentially converts the microUSB port of a device into a full-size port.
Right now, you can get one such cable from Amazon for $0.83. Yes, eighty-three cents. That's crazy cheap. Not only is this cable useful and affordable, but the shipping's free, too.