Geeks always want more and more from their devices. Even their device has limited features they still want more. USB OTG is another thing which is made for geeks.
A useful mean:
OTG allows you to attach several USB peripherals into your device through micro USB to full-size USB adapter. The basic idea is to use more input devices than a just touchscreen and limited on board storage. Suppose you are on a tight budget and you are thinking to buy a smartphone with nonexpendable storage then an OTG can save you a couple of bucks, just plug in a regular USB thumb drive and store whatever you want in it. You can use it right away anywhere. So by this way you can carry many USB drives according to your file needs.