The reason of range doppler terrain correction?

why do we have a step called “range Doppler terrain correction” in the workflow of sentinel 1 ??? , when using the sentinel 1 data in the agricultural classification.
I want the reason to know.

Because of the active nature of the system, every SAR image is acquired in slant looking geometry


If the ground is elevated because of hills and valleys, the time of the signal to travel to the earth’s surface and back to the sensor is distorted, causing to geometric shifts in the image: foreshortening, layover and shadow.
Examples are given here:

These can only be corrected if a model representing the topography under the image is known. Range Doppler Terrain Correction shifts all pixels to their correct locations according to an input digital elevation model.

The difference is demonstrated here:


If you have a completely flat surface, you could as well use a constant elevation (Average height RD) but in most cases, RD Terrain Correction increases the location accuracy of your image.