The time it takes for a radar signal to travel to an object and then scatter back to the spacecraft is used to determine the range between the spacecraft and that object. The slope causes responses from many locations to be returned at nearly the same time, however, so that they are bunched together into the same image location. The smaller a SAR's look angle, the greater foreshortening and layover distortions are seen in resulting images. There would theoretically be no such distortions for a look angle of 90 degrees. Radar illumination casts shadows just as the sun's rays do, and you can see that shadowing would grow worse as the look angle increased - as it does when the sun sets.
Many factors influence the choice of a look angle.