I *never* understood the idea of crop modes. Spend money on a x mp camera, and ask it to only use x/2 pixels. If it's about using the crop as a digital zoom, that could be far better done at the computer rather than in-camera.
It's quite simple really. All images are crops. A lens produces a circular image which is always cropped inside the camera to a rectangular shape of a particular format that matches the sensor.
Perhaps the confusion lies in the use of the terminology, 'cropped format'. All formats are in reality cropped formats. The full-frame 35mm format, or 135 format, which is 24mm x 36mm, is a cropped format. Perhaps the Canon APS-C format, or the Nikon DX format, should be called the 'cropped-more
A 75mp full-frame sensor, so called, when in APS-C mode, would be approximately 30mp, which is higher resolution than the latest Canon 70D APS-C. Effectively, you get two cameras in one.
Now, you might well ask why bother to put the camera in APS-C crop mode. Why not do the cropping during post-processing on the computer? Well, as Nancy mentioned, when photographing wildlife, birds, or any fast-action sport, you probably want the fastest frame rate and the largest buffer you can get.
For example, my D800E has a rather slow 4fps. However, in DX mode that rate jumps up to 6fps, and the buffer will also hold a greater number of images. Also, if one is shooting in continuous mode a lot, the fact that the memory cards will hold a greater number of DX-size images could be an advantage.