You're talking 2 different things. First of all, you can unplug the USB cable from the charger module and plug it into your laptop to transfer files to/from your Nexus 7. The Nexus 7 should show up as an additional drive in Windows Explorer.
The OTG cable is to allow you to plug in certain USB peripherals such as a USB thumb drive, mouse or keyboard. The OTG cable is more than just a cable with a female USB socket on one end, there is a slight difference in the wiring (2 pins connected together) that allow it to work. For the Nexus 7 to be able to read/write from/to a USB thumb drive or SD card you also need an app such as the Nexus Media Importer as well as the OTG cable.