What are the differences between down-looking and side looking radar?
A radar measures the time of arrival of the backscattered signal to infer the distance between the target and the platform (satellite, aircraft, whatever). If two targets are at the same distance from the platform, one on the left and one on the right because the platform is looking down, then their signals will arrive at the same time, making them impossible to distinguish. Looking sideways removes this ambiguity. Look at this figure from the Center for Space Research of the University of Texas (there is also a text explanation in the link).