interfaciing with a 5mp csi camera
I don't have the OV5640 datasheet in front of me but I can assure you won't be getting video over I2C.  Most sensors use I2C or SPI sometimes just to configure the sensor's many, many settings registers.   This control interface is also often used by the ISP (image signal processing) portion of the design, that attempts to make a pretty picture out of what you get back from the camera by adjusting all of those settings.

So short answer is I2C is for config the other interface is for image data which will be coming at a much higher rate.