Sometimes it makes sense to down-convert the image of your IP security camera to a lesser resolution than it's built to cap out at. The camera will have less work to do when encoding video. The data stream will be a smaller size eating up less bandwidth. The NVR will better be able to handle the encoding as well. You'll be the beneficiary of more available storage time with the same storage device. With higher resolution cameras you'll be able to record at a higher frame rate after lowering the resolution making the video more fluid.
There is a host of reasons why you might want to down-convert your resolution. Take note that using the Onvif driver for your camera may not allow changes to be pushed to the camera.